Retrieve password function

[JadeWang96]

Is your feature request related to a problem? Please describe.
If the user forgets the password when he/she sign in the system

Describe the solution you'd like
Add a simple function resets the password to a fixed code(sent to the user email), allowing the user to log in temporarily and changing the password.
Describe alternatives you've considered
More complex: send the temporary reset link to registered email and allow the user to change the password within 24 hours.

[rushsteve1]

So this is actually fairly complicated due to the nature of e-mail. But would 100% be something we should do since right now if you forget your email an Admin has to reset it.

[rushsteve1]

A useful list of possibly helpful libraries https://github.com/rust-unofficial/awesome-rust#email

[rushsteve1]

Generally we need a method of communicating with users.
Options to do this are:

• Write a Mattermost bot and ensure all users have their MM attached to their account
• Use an external email API like Sendgrid
• Host or use an SMTP server

[rushsteve1]

Issue superseded and blocked by #53