Improving the Joomla! extensions update for end-users

See Joomla! Tracker: TODO

# Summary

This Pull Request makes minor modifications to com_installer for the benefit of end users who have problems fetching updates and extension developers who no longer have to explain to users that they have an impossible to fix Joomla! problem and not a bug with their extensions.

The first change is that the Purge button is reinstated in the toolbar. Reasoning: an extension provides two subsequent updates in a time period less than the "Updates caching (in hours)" (default: 6 hours) setting in the Options page. The update will be invisible until these many hours have elapsed. Even if the user knows the update is there they can't see it and report it as a bug to the developers.

The second change is the addition of an Update Sites management page. Reasoning: Joomla! will unpublish update sites without warning if it cannot contact the update site's server. The user is not notified and the update site cannot be re-enabled. This means that a temporary network issue at the wrong moment will disable updates without warning, putting end users' sites at risk. Even worse, it was IMPOSSIBLE to re-enable the disabled update sites without editing the database. This was also reported to extension developers as a bug with their software.

These are the most often reported issues with the Joomla! extension updater by end users. Narutally the project was completely unaware of the high impact as the receiving end of the complaints has always been the extensions developers, not the Joomla! PLT. As an extensions developer I felt the urge to provide a fix to them for the benefit of everyone using or developing for Joomla!.

# Backwards compatibility

This PR is 100% backwards compatible.

# Translation impact

This PR introduces 11 new translation strings in 3 INI files. The average time to translate them is 3 minutes.

# Testing instructions

## Feature 1: Purge button

Log in to your back-end. Go to the Extensions, Extension Manager menu item. Click on Update. Before the patch the toolbar buttons on the left are only Update and Find Updates. After the patch there is a new third button called Purge.

Make sure there are updates shown on the page (you may want to install an outdated extension for that matter). Click on Find Updates. The updates list is cleared. Click on Find Updates. The update list is populated again.

## Feature 2: Update Sites

Log in to your back-end. Go to the Extensions, Extension Manager menu item. Look at the right hand link bar. Before the path the items are Install, Update, Manage, Discover, Database, Warnings, Install Languages. After the patch there is a new item called Update Sites. Click on it.

You see a list of update sites. Find an update site of an extension which currently has updates available and unpublish it. Go to Update, click on Purge and then click on Find Updates. You do not see the updates for the extension whose update site you just unpublished.

Go back to Update Sites. Publish the update site you had unpublished in the previous step. Go to Update, click on Purge and then click on Find Updates. You now do see the updates for the extension whose update site you just published