Add account change options for GitHub-linked users

[courte]

Based on issue #148 and dependent on issue #303:

Allow users who create a CodeMontage account with their GitHub account to set passwords. This could be as simple as sending new users who create a GH-based account an email to "set a password", should they wish to do so.

[courte]

I'm going to close this issue because it won't be relevant once #303 is solved.