Add start count corrector for Repository.NumClosedIssues

- Prevent panic on creating notice if database is not available
- Prevent incorrect folder on Windows ("/" instead of "\")

Fix #2872

Make user search function look in username (lower_name), full name
(full_name) and primary email (email). This will benefit searching after
user in "explore", admin panel and when adding new collaborators.

- Fix go-gogs-client#10
- Related to #809

Both are possible on explore and admin panel

Collaborators have write access as default, and can be changed via repository
collaboration settings page to change between read, write and admin.

- Allow to delete current avatar

Use hand-written SQL to do complex query

- Migrate: #2704 #2653
- Clone: #2701
- Mirror, Pull