Dynamic list width

[xbill82]

What does this PR do ?

This PR tweaks the width of the list views (only in the Data section, for the moment) in order to make them narrower on large screens (which seemed not very meaningful and ugly).
It also introduces some horizontal layout fixes to bugs that showed up in my 13' Mac screen.

How should this be manually tested?

• Open a collection on a large screen
• You should see that the document list (and the Watch section) are narrower than the window and have some whitespace on the right

Other changes

• Changing the width of the list containers also showed some layout weakness in some components. Some col s* m* l* xl* have been adjusted.

Boyscout

• Added the missing vue2-leaflet package
• Added some :key attributes to elements that had v-for

[codecov-io]

Codecov Report

Merging #448 into 2.x will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              2.x     #448   +/-   ##
=======================================
  Coverage   83.97%   83.97%           
=======================================
  Files          35       35           
  Lines         780      780           
=======================================
  Hits          655      655           
  Misses        125      125

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