Allow Users to Choose Date/Time Format

### Prerequisites

Please answer the following questions for yourself before submitting an issue. **YOU MAY DELETE THE PREREQUISITES SECTION.**

- [x] I am running the latest version
- [x] I checked the documentation and found no answer
- [x] I checked to make sure that this issue has not already been filed

### Problem Description

AdGuard Home shows dates as `3/21/2020` but I live in the UK so this format doesn't make sense to me, and I have to look twice at a lot of dates - there is no 21st month! Most people in the UK would expect `21/03/2020` - note the leading zero on single-digit values. However, I personally prefer a format like `2020-03-21`. There may also be people who prefer something like `21 Mar, 2020`.

Similarly for time, I would like to see seconds on all times in the AdGuard Home UI but I can see why others don't care about seconds. However, right now in the Query Log, AGH show seconds for items on the current day but not for items on previous days. Being able to choose this would be nice.


### Proposed Solution

Add an option in General Settings where users can choose the date format they prefer. Possibly also have a "Custom" option where users can specify the exact format they want, e.g. `yyyy-mm-dd hh:mm:ss` or similar.