Manually create todo for issuable
## What does this MR do?
Adds a button to the sidebar in issues & merge requests to allow users to manually create a todo item themselves.
## What are the relevant issue numbers?
Closes#15045
## Screenshots (if relevant)
See merge request !4502
Revert change to search all users
## What does this MR do?
Reverts a change that allowed the user to search for all users in the author/assignee dropdown
## Are there points in the code the reviewer needs to double check?
Double check it isn't still searching all users
See merge request !4564
Loads move issue dropdown async
To keep the style of the dropdown the same as the other dropdowns in the issue form, it uses select2 rather than our new dropdowns.
Closes#16563
See merge request !4160
Issuable filtering improvements
This improves the filtering of issues and merge requests by creating a single file that encapsulates all the filtering. Previously this was done with a file for issues and a file for merge requests.
Created the ability for the text search to be done alongside other filterables. Previously because this was outside the filterable form, this wasn't possible and would instead do either filter dropdown or text filter - not both. Fixes#4252
Fixed issue with not being able to filter and sort issues without refreshing the page. Fixes#15269
See merge request !3699
This button depends on Issue#can_be_worked_on? which in turn depends on
Issue#related_branches. By rendering this button asynchronously we can
finally remove all usages of Issue#related_branches and
Issue#referenced_merge_requests from the issues "show" page.
Phil Hughes
Stan Hu
Alfredo Sumaran
Jacob Schatz
Z.J. van de Weg
Grzegorz BizonandRobert Speicher
Yorick PeterseandJacob Schatz
James Lopez
