Add agencies de/select all in index.html

[jjimenezshaw]

The main change in this PR is the addition of two buttons: Select all and Deselect all for the agencies.

When you want to select just one agency, you did need to deselect manually all the checkboxes. 26 and counting. Now you can just deselect all and select what you are interested in.

It looks like this:

In addition to that, some minor css style changes were added:

• Checkboxes are always in front of their text, and not in two different lines
• A bit of responsive support for mobile devices:
  • meta viewport
  • style word-break in filenames (applies only if needed. Some are long)
• Margins more suitable for the increasing number of agencies (or narrow devices)

[rouault]

Thanks

[rouault]

@jjimenezshaw I do have an issue with Firefow 77.0.1: the agency line with checkbox is now on a single line , that goes beyond the width of my browser. There's a horizontal scrollbar that can be used to access the last checkboxes, but that's not really convenient.