If users are added to gogs due a reverse proxy automatically, their password is
set to their initial username. If their username is shorter than 6 characters,
they can never set a password. But since this password is required for some
operations (e.g. repository deletion), they could not use all features of gogs
until now.

This is because git command line shows the failure reason only if Content-Type is text/plain.

- code format for #1020

- fix delete user but repository watches are not decreased

Add setting.AppSubUrl in case of different ROOT_URL and LANDING_PAGE = explore

HTML tags are no longer processed for special links, etc
Contents of <a>, <code> and <pre> are not processed for special links
Processing for special links is done after Markdown conversion

- modules/middleware/context.go: add HandleAPI method