Reset captcha value

[degeri]

This will reset the the captcha to "false" after each sensitive request.
fixes #343

[jholdstock]

discussed on matrix:

If we want to force the user to complete a captcha for every action which sends an email, we dont really need to store CaptchaDone in the users session. Rather than flipping it between true/false, we can just assume it is always false
We still need c.Env["CaptchaDone"] so the template knows whether to draw the captcha or not, but we dont need session.Values["CaptchaDone"] at all

[degeri]

Looked at it again.

We cant remove session.Values["CaptchaDone"] right now cause there are are actually two requests 1. Captcha solve -> 2. Sensitive action, this requires that we "track" that a captcha is solved and then invalidate it such that it can only be used once.

For a "proper" fix we need to completely remove c.Env["CaptchaDone"] and session.Values["CaptchaDone"] and load up the captcha and the sensitive form in a single page / request (Would require some UX changes too). That needs be a whole another PR imo.

[jholdstock]

Discussed further on matrix. Need to set both env and session vars. Also, inside function SettingsPost() we need to set the vars in an else statement, to be consistent with the other two instances.

Should be good to merge after this.