Sort milestone drop-down list on issues page

[bugreport0]

• Gitea version (or commit ref): 1.5.1

Description

Milestones are sorted almost everywhere, but not in the drop-down filter box on the issues page.

To reproduce

1. Create a 'v2.0' milestone (assign a date like 2019-02-02)
2. Create a 'v1.0' milestone (assign a date like 2019-01-01)
3. Observe that milestones are sorted in the milestone overview and the 'set milestone for issue' list
4. Observe that milestones are not sorted in the 'milestone' drop-down filter on the issues page