Create join page

[altheaden]

This PR:

• Introduces a page for users to create an account, with some input validation (min and max lengths)
• Styles the aforementioned page, which includes modularizing the style from the login page
• Fixes an email link bug when sending a confirmation email
• Reorganizes the params/limits.py limits to include a dictionary for all text input limits, and passes this to the new template - this will likely be passed to other frontend templates as well

TODO: Still need to implement the "confirm password" functionality (text field exists but does NOT do any checking atm). I am making this PR anyways because that functionality seems to require using js and I haven't set up any js yet, so I wanted to hold off on that for now.

fixes #16

[altheaden]

@franklindyer I made that change, how do you feel about it?