This PR resolves Sub-feature (1.4) of Issue # 138:

• User templates available to be used within Ketcher2.
• Ketcher2 stores templates by default in browser's local storage.
• API endpoint is added to sync localstorage with the backend in the form of attachment/files.
• Now, user is able to add, remove, edit user template within ketcher2
• Demo Attached:

https://github.com/ComPlat/chemotion_ELN/assets/25753582/612006a5-0886-476c-8357-9c28e8046169

• [ ] rather 1-story 1-commit than sub-atomic commits

• [ ] commit title is meaningful => git history search

• [ ] commit description is helpful => helps the reviewer to understand the changes

• [ ] code is up-to-date with the latest developments of the target branch (rebased to it or whatever) => :fast_forward:-merge for linear history is favoured

• [ ] added code is linted

• [ ] tests are passing (at least locally): we still have some random test failure on CI. thinking of asking spec/examples.txt to be commited

• [ ] in case the changes are visible to the end-user,  video or screenshots should be added to the PR => helps with user testing

• [ ] testing coverage improvement is improved.

• [ ] CHANGELOG :  add a bullet point on top (optional: reference to github issue/PR )

• [ ] parallele PR for documentation  on docusaurus  if the feature/fix is tagged for a release

LCOV of commit ab58f8d during Continuous Integration #3017

Summary coverage rate:
  lines......: 64.1% (13691 of 21354 lines)
  functions..: no data found
  branches...: no data found

Files changed coverage rate: n/a

LCOV of commit 3e59bbe during Continuous Integration #3201

Summary coverage rate:
  lines......: 64.0% (13662 of 21345 lines)
  functions..: no data found
  branches...: no data found

Files changed coverage rate: n/a

LCOV of commit d43d75b during Continuous Integration #3390

Summary coverage rate:
  lines......: 65.2% (13637 of 20909 lines)
  functions..: no data found
  branches...: no data found

Files changed coverage rate: n/a

LCOV of commit 3cbdd3d during Continuous Integration #3482

Summary coverage rate:
  lines......: 65.2% (13638 of 20910 lines)
  functions..: no data found
  branches...: no data found

Files changed coverage rate: n/a

made redundant by #2061