DB console: remove the dropdowns for the hour:minute for time intervals

This concerns the "custom time intervals" dialog. The dropdowns that appear for the hour:minute are useless and extremely annoying. If you are using the keyboard, if you press Tab it goes through all the number options. You have to click outside the dropdown for it to close.. BUT if you click outside the dialog itself, it goes away completely and you have to start over, which is very frustrating.

The interval dropdown should also remember the few couple custom intervals used so you can just click on them. Without this feature, you have to manually enter it for every separate window you open (which is necessary if you're trying to look at multiple sets of metrics at the same time).

Jira issue: CRDB-47556