Implement remember login mode choice

[KGE]

Improves #269

Inspired by backend version

@LukeTowers I think this is a nice starting point

[manuelbua]

This would be a nice addition, any chance to have this merged upstream @LukeTowers?

[LukeTowers]

@manuelbua have you tested these changes locally? Could you send screenshots of the login form with the checkbox and the backend settings for this? Please test all three modes (Yes, No, Ask) and verify that they work correctly.

[manuelbua]

@LukeTowers, you explicitly asked for a PR resubmission and were also looking forward at merging it: i genuinely asked about it, because i thought it was going to happen.

[LukeTowers]

@manuelbua it is going to happen; however I don't have time to test this myself personally right this moment so I'm asking you to do it for me. Apologies if there was a miscommunication in what I meant to say.

[Eoler]

I tried it and it works (and most sensible setting is "ask", of course).
It goes to production site tomorrow... thanks, @KGE !

[Eoler]

This goodness still works with the latest plugin release, @bennothommo

[bennothommo]

@Eoler Are you saying that this PR works with the latest version of the plugin?

[Eoler]

Exactly, @bennothommo (ok, I didn't try 5 years expiry ;)
Its very handy with e-commerce sites, where security matters more...

[bennothommo]

Thanks for letting us know, @Eoler. I'll give it a test and merge it once reviewed.

[bennothommo]

@KGE I have tested this and it works very well - great job on this. I've merged in the master branch to resolve some conflicts, and also changed the "Yes" and "No" terms for the remember mode to "Always" and "Never", to make it more clear what the options mean in terms of setting a persistent session.