Problem: Cannot change "join request" instructions

[limdingwen]

Please state the bug, support or feature request in terms of a problem to be solved.

By default, if the server is set to requiring approval to join, it uses "This will help us review your application" as the text. However, a lot of servers will benefit from being able to change this text

(Optional) Proposed solution:

Current workaround would be to modify configs/locale/simple_form.<locale>.yml and rebuild Docker files, but this is not accessible to the average admin.

Proposing solution to add this to the admin web GUI. Need to figure out how to deal with multiple languages; perhaps ignore it if servers are commonly single-language anyway (i.e. if not set, then use the default locale-based message. If set, then use it regardless of locale.)

[weex]

Seems reasonable to me.

If we make this editable in the Administrator settings, it'd be similar to what's referred to as static settings in the code. https://github.com/magicstone-dev/ecko/pull/317/files is the latest PR that fixed some of those.