Create a 3.x version of microsites base

[markconroy]

I think we need to create a 3.0 version of the theme to support the new 2.0 version of LocalGov Base.

[msayoung]

It exists https://github.com/localgovdrupal/localgov_microsites_base/tree/3.x

Need to do a release.

[stephen-cox]

We need to update the composer dependency to the 2.x version of LGD Base.

[willguv]

@tonypaulbarker to test and release

[tonypaulbarker]

Notes - couple of commits to set up:

• localgov has this entry, i think we'll want a similar entry for microsites to release at the same time
  "localgovdrupal/localgov_base": "^1.3.1 || ^2.0.0",
• add similar upgrade notes to the readme as in localgov_base

[tonypaulbarker]

We have a PR set up for this, we are looking at Maria's note on alignment settings at #327

[tonypaulbarker]

@finnlewis I could only find a constraint for localgov_base to use 1.x in localgov_microsites_base 2.x composer.json, which should now be updated in 3.x to use 2.x

I think we can release 3.x in combination with a version update in the microsites project:
localgovdrupal/localgov_microsites#531

I have requested some review there.