Update legal, release notes, version to 1.4.0

[rly]

Prepare for release of hdmf-common-schema 1.4.0

• Updated legal years to include 2021
• Updated release notes
• Updated version to 1.4.0
• Minor updates to doc and GitHub PR template

NOTE: because of how HDMF-docutils is currently structured, only the HDMF-common namespace will be loaded and documented on readthedocs. The HDMF-experimental namespace is not loaded and documented.

Before merging:

• [x] Update requirements versions as needed
• [x] Update legal file dates and information in Legal.txt, license.txt, README.rst, docs/source/conf.py, and any other locations as needed
• [x] Update README.md as needed
• [x] Update the version string in docs/source/conf.py and common/namespace.yaml (remove "-alpha" suffix)
• [x] Update docs/source/conf.py as needed
• [x] Update release notes (set release date) in docs/source/format_release_notes.rst and any other docs as needed
• [x] Test docs locally (cd docs; make fulldoc)
• [x] Push changes to this PR and make sure all PRs to be included in this release have been merged
• [ ] Point the HDMF submodule to this branch in the HDMF branch corresponding to this schema version and check that HDMF tests succeed
• [ ] Check that the readthedocs build for this PR succeeds (build latest to pull the new branch, then activate and build docs for new branch): https://readthedocs.org/projects/hdmf-common-schema/builds/

After merging:

1. Create a new git tag. Pull the latest master branch locally, run git tag [version] --sign, copy and paste the release notes into the tag message, and run git push --tags.
2. On the GitHub tags page page, click "..." -> "Create release" for the new tag on the right side of the page. Copy and paste the release notes into the release message, update the formatting if needed (reST to Markdown), and set the title to the version string.
3. Check that the readthedocs "latest" and "stable" builds run and succeed. Delete the readthedocs build for the merged PR. https://readthedocs.org/projects/hdmf-common-schema/builds/
4. Update the HDMF submodule in the HDMF branch corresponding to this schema version to point to the tagged commit.

See https://hdmf-common-schema.readthedocs.io/en/latest/software_process.html for more details.