chore(ci): correctly deploy code and docs on version branches and tags

Previously, the generated build artifacts and docs were only deployed
for builds associated with the master branch. There was also a `latest`
branch mentioned in the config, but there is normally no such branch, so
this had no effect.

This commit fixes the rules so that deployments happen when necessary.
More specifically:
- The `deploy-code` job now runs for builds associated with:
  - The master branch.
  - The stable branch (i.e. the branch from which the version tagged as
    `@latest` on npm is released).
  - Tags of the form `v1.X.Y(-Z)`. (This also required configuring
    CircleCI to run builds for git tags, which does not happen by
    default.)
- The `deploy-docs` job now runs for builds associated with:
  - The stable branch (i.e. the branch from which the version tagged as
    `@latest` on npm is released).

The new rules for when deployments should take place are based on the
logic previously in [.travis.yml][1] and [scripts/travis/build.sh][2]
(from before we switched from Travis to CircleCI).

[1]: https://github.com/angular/angular.js/blob/974700af7c1/.travis.yml#L54-L103
[2]: https://github.com/angular/angular.js/blob/974700af7c1/scripts/travis/build.sh#L66-L101