Allow adding new projects just from URL

[Zlopez]

Description

Let's make adding projects easier. This is a feature that will allow users to just provide URL to project they want to add and Anitya should do the rest.

WORKFLOW:

1. When clicking on Add project menu entry the user will be redirected to a new page, which will ask him for URL to project he want's to add and click 'Add' button on page
2. User gets redirected to current Add project page prefilled with data obtained from project's page

Example:

1. User provides https://pypi.org/project/anitya/ and it will get redirected to /project/new with the project filled in with
   • name: Anitya
   • homepage: https://release-monitoring.org/
   • Backend: PyPi
   • Version scheme: Python
   • Ecosystem: pypi

Requirements

• New page that will allow user to put the URL
• New Flask view that will be called when submitting the URL, this will try to recognize the backend (the best way to do it is probably using domain name checking based on the backends - will probably need some new constant in each backend)
• New method in each backend (or at least in those where it's possible) that will obtain data from the URL provided
• The redirect to current Add project page with the data obtained in previous step (this will be probably part of the new Flask view)

How this will make Anitya better

It will make adding projects less tedious and quicker.