Various improvements to code snippets for groups

[rvisser7]

This PR adds some various improvements to the code snippets for Galois groups and abstract groups. In summary: Gap has been added to Galois groups, Oscar and transitive group snippets now added to abstract groups, and sidebar download links and snippet tests now added for all languages in both sections.

As always, any comments/feedback much appreciated! 🙂

• Added support for GAP in the code snippet testing CI (many thanks @havarddj for the help!).

• Added GAP code snippets to Galois groups, along with a sidebar download link for downloading the Gap commands, and added Gap snippet tests.

  http://localhost:37777/GaloisGroup/4T2
  https://beta.lmfdb.org/GaloisGroup/4T2

  http://localhost:37777/GaloisGroup/4T2/download/gap
  https://beta.lmfdb.org/GaloisGroup/4T2/download/gap

• Added new Oscar code snippets to abstract groups (thus partially resolving #6908), as well as also a few new Sage and Sage (using Gap) code snippets.

  http://localhost:37777/Groups/Abstract/6.1
  https://beta.lmfdb.org/Groups/Abstract/6.1

• Added code snippet downlink links for abstract groups in the right sidebar, and added code snippets tests (note that the "{lang} commands" download links are currently given at <label>/codedownload/<lang> , as the separate "Group to {lang}" constructions are already hosted at <label>/download/<lang> ).

  http://localhost:37777/Groups/Abstract/6.1/codedownload/gap
  http://localhost:37777/Groups/Abstract/6.1/codedownload/magma
  http://localhost:37777/Groups/Abstract/6.1/codedownload/sage
  http://localhost:37777/Groups/Abstract/6.1/codedownload/sage_gap
  http://localhost:37777/Groups/Abstract/6.1/codedownload/oscar

• I've had a go at adjusting and refactoring the code in groups/abstract/web_groups.py which generates the top code snippet for abstract groups (adjusting the original code I wrote during the LMFDB workshop last year). Since there's many cases to consider, this is now handled in a separate function called create_top_code_snippet. We also now use the code.yaml file to fetch code to construct particular families of groups (e.g. Symmetric, Alternating, Cyclic, Dihedral, etc.), instead of having the snippets hardcoded in the python files, therefore hopefully making it easier for others to add new languages in the future. (currently, the only slight exception to this is still the Lie-type code snippets. Since there's a lot of Lie families, and the Magma commands are already in the gps_families table, there's no Lie-type code in the code.yaml file).

  Links to examples of groups which test the many different top code snippets are given in this comment in PR #6579.

• Added code snippets for the transitive group constructions to abstract groups. This is implemented in the create_transitive_code_snippets function.

• I've also moved the code to generate the Lie type representation code snippets into the create_lie_code_snippets function. Since there can be multiple different Lie type representations and transitive group representations, and separate code snippets have to be generated for each one, I think it's easier to have the logic for these code snippets handled separately from the rest of the code snippets.