Cannot tab to form submit element or use enter to select month on alternate month picker

[morremeyer]

Describe the bug
In the month selector on browsers that do not support input type month, the user cannot select a month with the keyboard.

This is due to the form submit element not being tabbable nor the form being submittable by pressing the enter key.

To Reproduce
You need to use a browser that does not support month pickers, e.g. Firefox, to replicate this.

1. Set your frontend to dark mode
2. Go to "Home"
3. Click the calendar symbol in the top bar
4. Try to change the month without using your mouse or trackpad

Expected behavior
The month picker should support:

• Tabbing to the submit element of the form
• Pressing enter to submit

Screenshots

Desktop (please complete the following information):

• OS: macOS
• Browser: Firefox

Additional context

This also applies to the "Go to current month" link