Changing filter dropdowns shows loading
Instead of doing a full refresh of the page - i've modified the filterResults method on the Issues object to work for this form as well

Closes#14359
See merge request !3237
We're not concerning ourselves with non-HTML5 browser compatibility, and
this removes 21 KB from our compiled JavaScript.
Bonus fix: There was an extra space after the query string in the URLs
that has now been removed.
It is same search like we have at issues page. It allows to quickly
filter merge requests based on title or desription. I copy-pasted some
js code from Issues.js. In future search (filtering) logic should be
refactoed into one class for merge requests and issues