Change bridge line PT args to be space-separated.

 * FIXES #12932 https://bugs.torproject.org/12932
Bump Leekspin to 1.1.3; it now supports obfs4 mocking.
Add obfs4 to the HTTPS Distributor's PT options dropdown.

This is temporary. It is needed in order to make sure that the
regression tests for #12932 are functioning correctly. Because #12932 is
about the way PT arguments are presented to the user in the bridge
lines, to test the fixes for it, we must go through the UI (as the tests
in `lib/bridgedb/test/test_https.py` do).
Add obfs4 bridge line unittests and regression tests for #12932.
Fix an additional four bugs in bridgdb.Bridges.parseExtraInfoFile().

In addition to the one-character bug for #12932 which was fixed in
commit 487c1b6c16e979bc448ddd415364e28bd764d29a, there were an
additional four bugs in the legacy parser,
`bridgedb.Bridges.parseExtraInfoFile()` (which I am about to deprecate
anyway for #9380):

       # get the transport line
       if ID and line.startswith("transport "):
           fields = line[10:].split()
           # [ arglist ] field, optional
           if len(fields) >= 3:
               arglist = fields[2:]               # BUGS 1 and 2
               # parse arglist [k=v,...k=v] as argdict {k:v,...,k:v}
               argdict = {}
               for arg in arglist:
                   try: k,v = arg.split('=')      # BUG 3
                   except ValueError: continue    # BUG 4
                   argdict[k] = v
                   logging.debug("  Parsing Argument: %s: %s", k, v)

  BUG 1: This assumes the PT arguments are space-separated in the
         extrainfo descriptor. They are not; they are comma-separated.

  BUG 2: This would result in parsing the entire, comma-separated group
         of PT arguments into:

             {"key1": "a,key2=b,key3=c"}

  BUG 3: This would produce a ValueError, because there's more than one
         '=' character. (Meaning that the whole set of arguments would
         be discarded due to Bug #4.)

  BUG 4: The whole set of arguments gets discarded, without even so much
         as a log message, if there was more than one argument.

These are all bug fixes on a single commit,
4300329a30f3b6aa3e390b140193dd50faa6e03f, from #4568. And I'm still
deprecating the entire function anyway (for #9380) because the rest of
it is likely just as full of bugs.

 * FIXES #12932 https://bugs.torproject.org/12932
Disable obfs4 distribution (for now).

Until we have enough bridges supporting obfs4 to actually distribute
them, and Tor Browser has obfs4 built in, I'm leaving the HTTPS dropdown
item present but disabled, and nothing will be mentioned in emails sent
by the Email Distributor until the `"obfs4"` line in
`bridgedb.strings.CURRENT_TRANSPORTS is uncommented.
Merge branch 'fix/12932-pt-args-spaces' into develop