Preferred portal refresh

[mrcnski]

PULL REQUEST

Overview

• Implements redirecting the skapp to be on the user's preferred portal, if he has a preferred portal configured.
• Updates some URL utilities and tests based on edge cases that were found when implementing the redirect.

Note: Has not been tested yet. Waiting for the corresponding MySky changes, then will test on alpha MySky.

Full flow

 Load MySky:
 1. SDK opens MySky on the same portal as the skapp.
 2. MySky always connects to siasky.net first.
 3. MySky tries to get the saved portal preference.
    1. If the portal is set, MySky switches to using the preferred portal.
    2. If it is not set or we don't have the seed, MySky switches to using
       the current portal as opposed to siasky.net.
 4. After MySky finishes loading, SDK queries `mySky.getPortalPreference`.
 5. If the preferred portal is set and different than the current portal,
    SDK triggers refresh.
 6. We go back to step 1 and repeat, but since we're on the right portal now
    we won't refresh in step 5.

Login:
1. SDK logs in either silently or through the UI.
2. If it was through the UI, MySky switches to siasky.net and tries to get
   the saved portal preference.
   1. If the portal is set, MySky switches to using the preferred portal.
   2. If it is not set or we don't have the seed, MySky switches to using
      the current portal as opposed to siasky.net.
3. SDK queries `mySky.getPortalPreference`.
4. If the preferred portal is set and different than the current portal, SDK
   triggers refresh.
5. We go to "Load MySky" step 1 and go through that flow, but we don't
   refresh in step 5.

Checklist

Review and complete the checklist to ensure that the PR is complete before assigned to an approver.

• All new methods or updated methods have clear docstrings
• Testing added or updated for new methods
• Verify if any changes impact the WebPortal Health Checks
• Approriate documentation updated
• Changelog file created

[ro-tex]

Aside from the MySky singleton issue, it looks fine. Let's resolve that one and merge this.