Show prominent warning and howto in user and admin interfaces when user-initiated password reset is impossible

[Poikilos]

• Gitea version (or commit ref): 1.9.2 (Go1.12.9)
• Database (use [x]): unknown (admin unavailable)
• Can you reproduce the bug at https://try.gitea.io:
  • Yes (provide example URL)
  • No
  • Not relevant
• Log gist: N/A

Description

User-initiated reset says it is disabled, when in reality, e-mail is probably not configured, according to comments at #2247 which is closed for unknown reasons which the closer did not describe.

Expected behavior:

• Provide warning message such as proposed by jolheiser. Whereas "disabled" is misleading (according to replies at which indicate that the message only appears when e-mail is not configured and that there is no setting for it), "disabled because e-mail is not configured" (maybe that on that admin side, and something like "is not configured" on the user side) would be more clear, and would point to the solution.
• Where the warning is shown prominently, also explain how to correct the issue (at least on the admin side).

[jolheiser]

Fixed by #15117, which shows a better error to admin as well as logging a warning when that page is shown (so an admin looking for something in logs can maybe still find it)