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