Fix form to add exercises to days

[adrianlzt]

Proposed Changes

• Fix the removing of a prev form if sets are changed
• Closes #1553

Please check that the PR fulfills these requirements

• [ ] Tests for the changes have been added (for bug fixes / features)
• [x] Added yourself to AUTHORS.rst

Other questions

• Do users need to run some commmands in their local instances due to this PR (e.g. database migration)? No

Adding an exercise with id > 999:

Default form:

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After moving the slider: