Redirect people to intended destination after login

[suecarmol]

Description

Added a next_url parameter to preserve original link clicked before login.

Rationale

People want to be redirected to the original link they clicked, not My Library

Phabricator Ticket

How Has This Been Tested?

Tested manually

1. Try opening a login_requiredlink or a proxy link in a session where you are logged out
2. You should be redirected to the homepage with the message To view this link you need to be an eligible library user. Please login to continue.
3. Login
4. You should be redirected to the proxied link or the original link you navigated to

Screenshots of your changes (if appropriate):

Types of changes

What types of changes does your code introduce? Add an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Minor change (fix a typo, add a translation tag, add section to README, etc.)

[jsnshrmn]

Looks great, works great!