[issue_tracker] Enable editing of site, assignee and watchers in Batch Mode

[ay-bh]

Brief summary of changes

• Added site selector to Issue Card edit mode
• Enhanced Add Comment modal with assignee and watchers selectors
• Updated backend endpoint to fetch otherWatchers and assignees

Testing instructions

1. Go to Issue Tracker Batch Mode
2. Click "Edit Issue" on any issue
3. Verify site can be modified using the new dropdown
4. Update Issue
5. Click "Add Comment"
6. Verify assignee and watchers can be modified in the modal
7. Submit changes and verify they persist

Link(s) to related issue(s)

• Part of [issue_tracker] batch mode - follow up #9418

Screenshots

[christinerogers]

@ay-bh if this is ready for review -- please request review from @regisoc and @racostas

[regisoc]

Thanks for this PR, comments:

• Some static test did not pass.
• adding/removing watchers looks good and in sync when switching from edit/detail view to batch mode.
• site dropdown, I tried to change to No Site but had the following image show up.
  This option should not be added to force user to choose one site.

• issue: unassigned state card text. When no assignee or unassigning someone from an issue, there is a blank in the card. This should instead have an explicit "unassigned" tag. Also, on a second thought, it is in line with the previous comment, not allowing to have a ticket unassigned might be a good idea. The goal of issue tracker is to resolve those tickets. Not having someone assigned, they will be forgotten.

• issue maybe? @racostas I think it is not linked to this PR: when updating the assignee, no history about this change in the edit/detailed view. The change is acted on the batch mode card view, and in the edit/detailed view but not in the history section.

[regisoc]

Thanks for this update @ay-bh :

• No site issue: is ok, option removed.
• Unassigned Issue : states Assignee:None in card. Front-end looks ok. I am scratching my head here as I see it as mandatory as the previous point. In HBCD, we enforced a default assignee based on category so that issues do not fall into limbo. @christinerogers @racostas does that make sense to you that an issue is not assigned?
• no assignee update in history issue: could/should be referenced in another issue ticket if necessary.

@racostas @christinerogers If the second point is ok to you, then the PR looks ok to me.

[christinerogers]

Thanks @regisoc for these comments.
from the bottom up:

• no assignee update in history issue: could/should be referenced in another issue ticket if necessary.

Agree this should be another issue, and it's important. I'll add this to the Followup ticket #9418. @ay-bh if you feel keen to address this on your own time, thank you - it's optional since you have completed GSOC.

• No site issue: is ok, option removed.
• Unassigned Issue : states Assignee:None in card.

@regisoc I get why HBCD has customized this module to make assignee and site selection mandatory, and there are valid reasons these were not mandatory in the Loris module design.
Let's leave this as it is currently, keep the no-site enforcement change, and I'll add a follow up point in #9418 to make both of these configurable parameters.

@racostas @christinerogers If the second point is ok to you, then the PR looks ok to me.

Agreed let's proceed with this PR as is. Over to @racostas for review to move this PR forward.

Note this no-site design change requested by regis should be documented in the module readme, and this is one of many points to follow up on in #9418

[regisoc]

@racostas waiting for your review, I will put you as assignee.