Fix sending feedback for recurring events

[seluianova]

Short description

Slugs for recurring events contain date (in format: base-slug$yyyy-mm-dd). As a result, when we receive a feedback for recurring event, event can't found by slug in DB and response 404 is returned.

Proposed changes

Remove the date from the slug during feedback processing

Side effects

• If someone manually creates event with the slug ending with the date in format $yyyy-mm-dd, feedback for such event won't be processed (upd: actually not possible, symbol $ is not allowed in the slug)
• There is no validation of the date in slug. If we receive feedback for an existing event but with an nonexistent date, it will be processed successfully.
• We lose information about which date the feedback belongs to.

Resolved issues

Fixes: #1793

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