HSTS: Rewrite the HSTS feature and add testshsts

This is a comprehensive rework of the HSTS enforcer and related
classes, based upon Adrien Plazas work. A summary of the most
relevant changes:

SoupHSTSEnforcer:

- The enforcer will listen on headers both on message queueing
  and restarting. This is necessary in order to be able to enforce
  HSTS redirections on messages that are restarted for whatever
  reason.
- Instead of causing a redirection, the URI will be overwritten
  directly on the message before this one is sent. Redirections
  are for use on the server side, and the tests added show that
  it is not a reliable way to do HSTS enforcing. Currently,
  the only way to find out that a HSTS policy has been enforced
  is by listening to the SoupMessage:uri property changes, but
  this might be impractical, so this could be revisited in the
  future.
- soup_hsts_enforcer_policy() will not steal the given policy.
  Doing so is prone to leaks and not customary.
- SoupHSTSEnforcerClass now has a has_valid_policy() vfunc.
  It currently works exactly as before, but the idea here is to
  make it possible for subclasses to implement their own check
  for existence of valid policies for domains, instead of all
  subclasses having to add their policies to the base
  SoupHSTSEnforcer class. This will be useful when having a large
  number of pre-loaded HSTS policies (either in SoupHSTSEnforcerDB
  or in an enforcer using libhsts as a backend) to avoid having
  potentially thousands of policies in memory at all times.
- HSTS headers are parsed using soup's available utilities,
  instead of parsing them by hand. The specification is carefully
  followed so as to not accept any header that is not fully
  compliant.

SoupHSTSEnforcerDB:

- Store the max-age attribute in the database. This was done before
  errata 5372 was reported to RFC 6797, and its necessity will
  depend on how the errata is treated.

Other:

- Added tests for both enforcer classes that cover most of the
  specification.
- Added the gtk-doc documentation and update all the documentation
  comments.
- Rename SoupHsts classes to SoupHSTS for consistent naming and
  other minor renaming of parameters and methods.

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