Add session-tab icon numbers

[cezaraugusto]

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

Auditors: @bsclifton, @bbondy

Fix #7367
Fix #3083

Note: this visually blocks sessions higher than 9 due to UI limitations.
/cc @bradleyrichter

Test Plan:

npm run test -- --grep="should show partition number for new sessions"
npm run test -- --grep="should show max partition number even if session is bigger"

[bbondy]

Pls remove this file and instead just put the value in js/constants/config.js

[bbondy]

This is really great, thanks @cezaraugusto. Please do submenus too for this version. See Slack convo with @bradleyrichter. There's one nit only but I'm merging independent of it.