Merge branch 'ui/issuable-filter' into 'master'

UI: Issuable filter tweaks

Depends on !1953

See the commits for more details, the messages mostly speak for themselves.

# Highlights

## Filter bar

Before:

![filter_before](/uploads/b061be7521c6d5babb77c7d12e77f65e/filter_before.png)

After:

![filter_after](/uploads/aadfbcd77caf1b49fde8ca6d781f1004/filter_after.png)

## Bulk edit bar

Before:

![bulk_before](/uploads/849c98224e1f335c65c0de5616bbcdae/bulk_before.png)

After:

![bulk_after](/uploads/185d29116663f9f663acab76d328096f/bulk_after.png)


See merge request !1963