Tomtoolkit

This version includes the following:

- Removes support for Python 3.6, and upgrades ``numpy`` to 1.20, ``astropy`` to 4.2, and ``astroplan`` to 0.8.
- Refactors ``user_list.html`` into templatetags to support introduction of installable app ``tom_registration``.

This release addresses the following:

- Retains the selected tab on reload for target detail page, as well as selected filters on target list and observation list when updating statuses. (427)
- Adds new statuses supported by the LCO facility. (409)

What to watch out for

- In order to leverage the tab retention, you'll need to copy the changes in ``tom_targets/target_detail.html``, ``tom_observations/observation_list``, and ``tom_targets/target_list.html``. You can review [this pull request](https://github.com/TOMToolkit/tom_base/pull/436/files) to see what has changed.

This release fixes the erroneous use of `photon_flux` and replaces it with `flux` for spectroscopic data processing. It also fixes past usage of `photon_flux` using a migration script.

This release will require running `./manage.py migrate`, and will fail if you have instances of `NaN` in the database.

This release fixes the ObservationRecordCancelView and adds a path to it to `urls.py`. It is now accessible via `tom_observations:cancel`.

This release fixes the following issues with dynamic cadences:

- Ensures that an exception in one dynamic cadence does not stop the processing of remaining cadences.
- Attempts to restart cadence in the case that a window falls behind.

This release adds RESTful API endpoints for submitting, viewing, and listing observations.