release-23.2: ui: add static images to asset build step

[blathers-crl]

Backport 1/1 commits from #121380 on behalf of @dhartunian.

/cc @cockroachdb/release

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During the genassets build + embed step, we were taking just the output of the db-console-ccl or db-console-oss step which is just a build.js file. This commit adds references to the image assets we want bundled as well. This includes favicon.ico and everything in ./ assets relative to the db-console build directory.

We disable content hashing in webpack in order to keep the filenames static, which bazel requires. The impact should be minimal as we rarely change these images so if they're cached forever, it's fine.

This change restores the favicon to the CRDB build and the nice image that shows up in the background of the email signup bar.

The size of the final zipped bundle only differs by around 1MB and is already 10MB in size.

Fixes: #117876
Epic: None

Release note (ui change): the favicon now renders properly for DB Console along with other image files.

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Release justification: low-risk high impact change to ui

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
• Backports should not break backwards-compatibility.
• Backports should change as little code as possible.
• Backports should not change on-disk formats or node communication protocols.
• Backports should not add new functionality (except as defined
  here).
• Backports must not add, edit, or otherwise modify cluster versions; or add version gates.
• All backports must be reviewed by the owning areas TL and one additional
  TL. For more information as to how that review should be conducted, please consult the backport
  policy.

If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.
• Your backport must be accompanied by a post to the appropriate Slack
  channel (#db-backports-point-releases or #db-backports-XX-X-release) for awareness and discussion.

Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is 

[dhartunian]

Tested local build: