Fire signal when password verification fails

[CodeRhymesLife]

Addresses #214 where there is no way to be notified when a user attempts to login with an incorrect password.

These changes:

• Create a signal for invalid password
• Fire signal when user's password is invalid
• Update documentation

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@a379fa0). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #230   +/-   ##
=========================================
  Coverage          ?   90.64%           
=========================================
  Files             ?       25           
  Lines             ?     1454           
  Branches          ?        0           
=========================================
  Hits              ?     1318           
  Misses            ?      136           
  Partials          ?        0

Impacted Files	Coverage Δ
flask_user/signals.py	100% <100%> (ø)
flask_user/forms.py	98.12% <100%> (ø)

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[jameshanlon]

@lingthio Can this be merged yet?!

[and-semakin]

Hi @CodeRhymesLife! Thank you a lot for your contribution! It's a really important functionality. I like that you updated docs and added a recipe :) I just have a suggestion before we can merge it.

[and-semakin]

I think that more appropriate place for this code is somewhere around here. Actually user_logged_in is sent from that file too. So to not place this events (user logged in and authentication failed) too far from each other I would prefer both of them to be fired from user_manager__views.py.