Disable autocomplete when adding new timeframe

[bobapple]

Describe the bug

Filling a textual datetime description for Start and End of a new Timeframe results in an autocompleted absolute date. To workaround this I added the Timeframe directly to the database.

To Reproduce

Steps to reproduce the behavior:

1. Create new Timeframe
2. Add -30 days for Start
3. Move curser to another input field
4. Start becomes 12/1/2021, 12:00 AM GMT+1

Expected behavior

The textual datetime description should remain.

Your Environment

• Module version: 0.10.0
• Dependent module versions:
• Icinga Web 2 version and modules (System - About): 2.10.1
• Web browser and version:
• PHP version used (php --version): 7.4.3
• Browser: Google Chrome Version 101.0.4951.64 (Official Build) (x86_64)

Additional context

This may be due to a browser "feature", but perhaps this can be disabled.