[web] Improve Section component (former Category)

[dgdavid]

Problem

The Category component needs some improvements now that we have pages too, not only the overview.

Solution

To change a bit the component for fulfilling current needs.

Starting with a rename, the Section component now allows

• To be used without icon (maybe useful for a settings pages with a lot of sections)
• To specify a description for helping the user to understand what is the section about
• To include a visual separation between its title and the rest of its content
• To put an action right after the title, usually for opening a settings popup or similar.
• To change the icon for mentioned action (a cog by default)
• To include a tooltip to be shown when user hovering the section action

Screenshots

Below, see a few examples by using just fake sections

Not displaying the tooltip	Displaying the tooltip

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Related to #321

[coveralls]

Coverage increased (+0.1%) to 74.696% when pulling 6012248 on improve-section-component into 49e6a2a on master.

[joseivanlopez]

LGTM

As discussed offline, it would be better to use composition instead of passing props, for example:

<Section>
  <SectionAction />
</section>

And the same for the Fieldset created in #333:

<Fieldset>
  <Legend />
</Fieldset>

Agreed to change it in a refact PR.