Maintainers should be allowed to add environment variables at a project level

[seanhamlin]

Describe the bug

From https://docs.lagoon.sh/administering-lagoon/rbac/#maintainer

Maintainer
The maintainer role can do everything within a group and its associated projects except deleting the project itself or the production environment.

Yet when a maintainer attempts to update the environment variable at a project level, they can delete it, just not add it.

To Reproduce

Steps to reproduce the behavior:

% lagoon add variable -p example --name REDIS_HOST --scope runtime --value 'banana.0001.apse2.cache.amazonaws.com'
Error: graphql: Unauthorized: You don't have permission to "project:add" on "env_var".

% lagoon delete variable -p example --name REDIS_HOST                                                               1 ↵
✔ Yes
Result: success

% lagoon add variable -p example --name REDIS_HOST --scope runtime --value 'banana.0001.apse2.cache.amazonaws.com'
Error: graphql: Unauthorized: You don't have permission to "project:add" on "env_var".

Expected behavior

Maintainers can add environment variables at the project level

[grahamethompson]

This would be very helpful.