[IMPROVED] Allow e-mails only for login

This is a major change in MPOS. Usernames will not be allowed anymore.
This will avoid a lot of brute force issues since usernames are not a
valid login method anymore.

Fixes #1345 once merged.
[IMPROVED] Dropped username from login
[UPDATE] MPOS version due to major change
[IMPROVED] Adds Email label and removes maxlength

When logging in from mobile, there currently is not an indicater to
use email or username. This labels it correctly.

Removed maxlength to allow for lengthier email addresses.
pplns payouts speed improvements / reworked insert method
Merge pull request #1351 from TheSerapher/issue-1345

Issue 1345