Add column type targets to API

[mathemancer]

Related to #201

This adds a property to the columns endpoint giving the valid target types to which the type of a given column can be altered. This is a list of DB-layer types in the valid_target_types field of a given column in the /api/v0/tables/<table_id>/columns/ endpoint (also available from a column detail view). The string representations listed can be fed directly as arguments to update a column type.

Technical details

As a side benefit, this PR uses the MathesarColumn object as the representation of columns for viewing in the API. This gives more flexibility than using the raw Column type from SQLAlchemy.

Screenshots

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the master branch of the repository
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

Developer Certificate of Origin

Developer Certificate of Origin

Developer Certificate of Origin
Version 1.1

Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
1 Letterman Drive
Suite D4700
San Francisco, CA, 94129

Everyone is permitted to copy and distribute verbatim copies of this
license document, but changing it is not allowed.


Developer's Certificate of Origin 1.1

By making a contribution to this project, I certify that:

(a) The contribution was created in whole or in part by me and I
    have the right to submit it under the open source license
    indicated in the file; or

(b) The contribution is based upon previous work that, to the best
    of my knowledge, is covered under an appropriate open source
    license and I have the right under that license to submit that
    work with modifications, whether created in whole or in part
    by me, under the same open source license (unless I am
    permitted to submit under a different license), as indicated
    in the file; or

(c) The contribution was provided directly to me by some other
    person who certified (a), (b) or (c) and I have not modified
    it.

(d) I understand and agree that this project and the contribution
    are public and that a record of the contribution (including all
    personal information I submit with it, including my sign-off) is
    maintained indefinitely and may be redistributed consistent with
    this project or the open source license(s) involved.

[kgodey]

@mathemancer this looks great!

The only change I'd request is to add the column index to the ColumnSerializer. In the current flow, when I looked at the list of columns, I couldn't figure out what the index was to get the PATCH URL for the column detail view that I wanted to alter except by counting the list manually.

[kgodey]

Please also resolve merge conflicts.

[kgodey]

@mathemancer I would just name the field index, it's already attached to a column so the column_ seems redundant.

[kgodey]

Looks good, please merge once you rename the serializer field.