admin: Update template tags

Add Django template tag that returns the filter type for a given filter
and add a choice template filter that returns a list with the available
choices for a filter (only for MultipleChoiceFilter types).

Also, use an array of filters instead of generator in the templatetags
context, since the latter cannot be re-iterated.
admin: Use abbreviations for all model filters

Normally, we would like to name our model filters more verbosely: e.g.
proj --> project, net --> network. However, django-filter fails to
understand that the filter we create has nothing to do with the actual
model field and crashes, if defined like so:

    project = django_filters.ChartFilter(..., action=my_action)

If we try to circumvent it like so:

    proj = django_filters.ChartFilter(..., name='project',
                                      action=my_action)

then no results are printed since the query value gets incorrectly
cleared by the FilterSet form.

There doesn't seem to be an easy or clear way to circumvent this issue
so, for now, use as filter names the abbreviations of their model names
in order to be consistent across views.
admin: Conditionally concatenate filter terms

Concatenate filter terms of model filters that have the "=" sign between
them. Also, add a setting (`ADMIN_FIELD_SIGN`) for the administrator to
change that sign if he/she wants to.
admin-ui: Add Filters' Compact View HTML structure

* Show filters area only when the corresponding view has filters.
* Update Tips and Tricks section.
* Add extra instructions in Compact View area.
admin-ui: Add style for Filters' Compact View

Add only SASS files.
Add generated CSS files
admin-ui: Extend tables.js

Add functionality for Filters' Compact View.
docs: Move Archipelago setup to Node 2 prerequisites

Move Archipelago setup to Node 2 prerequisites on install guide.
docs: Update debian install guide

Add Archipelago-imposed limitation to run gunicorn workers as root.
Merge branch 'release-0.16-admin-filters-compact-view' of https://github.com/AthinaB/synnefo into release-0.16
docs: Update BlockStorage API guide

Update the BlockStorage API guide to properly showcase the GET
/snapshots action.