Stop trying to guess the server URL

Situation

When adding an account the app is currently trying to understand the given URL by deconstructing and reconstructing it using different patterns. It is used for instance when one enters "127.0.0.1" in the server URL field: then several URLs will be tested and the one giving a good answer is used. Several HTTP and HTTPS patterns are used. See compute_urls() for technical details.

This leads to several issues:

• If an URL answers successfully but it is not the Nuxeo server behind, a message asking for a downgrade to Nuxeo Drive 3.1.2 will be promtped.
• As seen with NXDRIVE-1541, Kaspersky is kind of intercepting HTTP calls and kill + delete the app when 8 calls are done. This is unfortunate ...

So trying to be smart is a nice developer win, but a poor end-user feature (for that specific case).

Scope

• Remove guess_server_url() and compute_urls() functions.
• When one is entering an URL, a real-time URL vlidator will ensure the URL is OK (if not, the OK button will be locked).
  This is a simple syntax checker that tells the text entered is a valid URL. If the URL is valid but not good, it is up to the user to do things right.
• The server URL field placeholder has to be updated to reflect the desired URL syntax.

A valid URL fills those requirements:

1. Starts with "http://" or "https://";
2. then comes the domain name "a.b.c.d.";
3. then comes the eventual port number ":8080";
4. then comes the default domain "/string";
5. then comes eventual key=value paramerters '"?param=value;param2=value2";
6. then comes the eventual anchor "#NXDRIVE-NEXTGEN2"