Introduced support for only 1 tab/window behaviour

For some situations like on the kiosk-mode, it's neccesary that
all the navigation is done on the same window and tab, which would
be opened fullscreen and without controls to switch between tabs,
so everything should be displated in the same view.

A new setting is introduced as part of the lockdown system to
enable this behaviour.

https://bugzilla.gnome.org/show_bug.cgi?id=703648
Merge the different error pages

The different epiphany HTML error pages have been refactored into
a single one, that depending on the type of the conditions include
some different styles to keep the original appearances.

https://bugzilla.gnome.org/show_bug.cgi?id=705180
Add new home/back buttons to standard error page

Adds new buttons to go back (if navigation is not locked down)
and return to the home page (the overview or the configured
homepage-url if its corresponding gsetting is defined).

https://bugzilla.gnome.org/show_bug.cgi?id=705180
ephy-string: Avoid poking at remote websites

Epiphany tries to query the existence of remote sites before loading
their page. This causes problems when the remote site is not available
and loading it requires the query to timeout (epiphany would look like
it's not loading the site for up to 60 seconds) or for one-time URLs
(where it might invalidate the query, such as for password resets, or
confirmation e-mails).

We'll now only check whether the file exists so that incomplete URLs
("epiphany foo.com/bar.html") still loads the bar.html file if it exists
locally, but the remote website if not.

https://bugzilla.gnome.org/show_bug.cgi?id=703772