Configure firefox-esr for mobile and privacy

Screenshots:

• homepage (1, 2)
• hamburger menu
• preferences page at 50% (1, 2, 3)
• pmos website
• pmos wiki
• toolbar menu
• homepage as thumbnail in phosh

Main commit description:

main/postmarketos-config-firefox: new aport

Add policies.json and prefs.js to configure Firefox (yes, you need both
to be able to control all this):

Mobile improvements:
* Enable zooming with fingers
* Mobile user agent (from tor browser for android)

Privacy improvements:
* Disable search suggestions, so URLs do not get sent to search engines
  as they are getting typed.
* Disable Firefox studies
* Disable Telemetry
* Set DuckDuckGo as default search engine, not Google

Uncluttering (screen space, remove broken features, less distractions):
* Move all buttons to the overflow menu and remove spacers around the
  address bar
* Empty "new tab" page (loads faster, no annoying "top sites" etc.)
* Disable developer tools, so the hamburger menu fits the screen of the
  pinephone. These aren't really useful on phones anyway.
* Disable "Firefox Screenshots": the feature did not work with the mobile
  resolution in Phosh.
* No default bookmarks from Firefox
* Disable First Run Page
* Disable Post Update Page
* Disable "User Messaging" (What's new, Extension/Feature
  Recommendations, Urlbar Interventions)

Add a local html file that serves as lightweight default homepage. It
links to the postmarketOS blog and wiki, explains that this is desktop
firefox with mobile configuration and explains how to change settings
and install addons (with a direct link to ublock origin). Users can
override the homepage, new tab page, search engine, search suggestions
pre-configured by this package directly in the UI.

I chose Firefox ESR, because:
* It fits the screen better: the "new tab" button is bigger, the menu
  opening when you click the top-right button actually fits the screen,
  with all its submenus. There are zoom controls in the same menu, which
  make the preferences pages very usable.
* It's possible to override the default search engine with a policy
  file; that's not possible in non-ESR firefox (we'd probably need to
  change it in the sources).

Related: https://github.com/mozilla/policy-templates
Co-authored-by: Martijn Braam's avatarMartijn Braam <martijn@brixit.nl>

Important take-away: it's easy for the user to override almost everything configured here, directly through the UI. The start page explains how to do so, even for casual users.

Other commits:

main/postmarketos-ui-phosh: add firefox config

Replace firefox with firefox-esr, remove epiphany. With the current
version of epiphany, the postmarketOS wiki doesn't even load every time,
so that's not something we want to ship.

main/postmarketos-ui-plasma-mobile: add firefox config

While at it, sort _pmb_recommends alphabetically.

[ci:skip-vercheck]: only _pmb_recommends changed

Based on !1440 (closed) (thanks, @MartijnBraam!)

Close: #670 (closed)