fix: Add doc on the TM database schema as it exists now

docs/tmschema.md

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+# Tasking Manager Database Schema
+
+The TM database schema has evolved over the years. It is currently
+implemented using sqlalchmey for the backend.
+
+## Misc Tables
+
+* alembic_version - The version of alembic used
+* banner - A list of banners, which doesn't appear to be used anywhere
+* priority_areas - A list of geometries for priority areas
+* release_version - The version of the TM release
+* spatial_ref_sys - Postgis support
+
+## Prepopulated Tables
+
+These tables contain data used by the frontend mostly. These could
+have been an Enum, but a tables can be updated by the front end,
+whereas an Enum can't. It does seem entirely possible these could be
+extended by a project manager, for example, adding a new license.
+
+* interests - General mapper interests
+* licenses - Data licenses
+* mapping_issue_categories - Issues with the map data
+* application_keys - Application keys for remote data. This appears to
+  not be used.
+
+## Campaign Tables
+
+The campaigns table is for mapping campaigns. A single campaign can
+involve multiple organizations and TM projects. 
+
+* campaigns - Primary table
+* campaign_organisations - Utility table to relate organizations with
+  a campaign
+* campaign_projects - Utility table to relate projects with a campaign
+
+## Organization Tables
+
+This is for an Organization profile.
+
+* organisations - Primary table
+* organisation_managers - Utility table to relate managers with an organization
+
+## Project Tables
+
+A project is the area to be mapped for the campaign. Each project
+contains members and teams, which are stored in other tables.
+
+* projects - Primary table
+* project_allowed_users - Utility table to relate users to projects
+* project_custom_editors - Utility table to store custom mapping
+  editors, currently only used by RapidID
+* project_favorites - Utility table to store favorite projects for a user
+* project_info - Details on the project, like the description & instructions
+* project_interests - Utility table to relate projects to interest categories
+* project_priority_areas - Utility table to relate priority areas to projects
+* project_teams - Utility table to relate team roles to projects
+* project_chat - Support commenting for a project
+
+## Messages Table
+
+Support messaging between users.
+
+* messages - Primary table
+
+## Task Tables
+
+A task is the area for a mapper to map. Task management is obviously a
+key function of the Tasking Manager.
+
+* tasks - Primary table
+* task_annotations - Utility table for something, it apperas to be
+  unused anywhere
+* task_history - Table for task history
+* task_invalidation_history - Table for task invalidation history
+* task_mapping_issues - Table for issues with a task
+
+## Team Tables
+
+This is for OSM Team support.
+
+* teams - Primary table
+* team_members - Utility table for team member profiles
+
+## User Tables
+
+* users - Primary table
+* user_interests - Utility table to relate interests for a user
+* user_licenses - Utility table to relate the data license to user
+* users_with_email - Utility table of user email addresses, which
+  appears to be unused
+
+## Notification Table
+
+This is a simple system for notifications. 
+
+* notifications - Primary table