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

Improve test cases, config settings, also show SSH config settings on admin config panel.

Moving cache variable to template instead of in the struct.