[Issue Tracker] Automatically specify Assignee/Watcher based on Category

[SantiagoTG]

For the Issue Tracker, add a button/functionality to 'Add specialists as assignee/watchers', such that if a ticket has a given Category selected, designated specialists for the category are automatically set as assignee/watchers.

Field (either a button or dropdown):

• 'Add specialist as assignee'
• 'Add specialists as watchers'

e.g. If Category chosen is 'Imaging', and the 'Add specialists as watchers' button is checked, the designated 'Imaging specialists' would automatically be set as watchers. Likewise for the assignee.

Related to:

• https://redmine.cbrain.mcgill.ca/issues/11048
• https://github.com/aces/Loris/issues/4150
• https://github.com/aces/Loris/issues/8328

[regisoc]

@SantiagoTG Reference to #8899 as same question could be on the other issue: are site/category the only criteria to be taken into account? More to be aware of or to come?

Also, is there an order between those criteria (e.g. site>category) to avoid inconsistencies? Example of issue:

An issue has the category A on site B. 
Person P1 is the default on category A. 
Person P2 is the default for site B.
Who is selected as default (P1 or P2)?