Feature/override selected block sets

[shipurjan]

Hello, this is a PR for a feature requested here (I needed it too)
#367

Basically it introduces the UI from Lockdown for Override, so that Override can be used for selected Sets and not only globally.

Use case:
I want to use LeechBlockNG to have some websites (defined 1 website per 1 block) blocked at all times, and only override them temporarily when I decide. But with current global override, it would give me access to all websites, not just 1 that I need. So with this solution it should work similar to Lockdown mode, but reversed.

Here are the test cases that I tested with this PR

#	Test Case	Expected Result
1	Open Override with no block sets having "Allow temporary override" enabled	Shows error: "No block sets have 'Allow temporary override' enabled"
2	Enable "Allow temporary override" for 2+ block sets, open Override	Shows checkboxes only for those sets
3	Select 1 set, enter 5 mins, activate	Override activates for that set only
4	Select multiple sets, activate	Override activates for all selected sets
5	Click "Activate" with no sets selected	Shows error: "Please select the sites to override"
6	Click "Activate" with no duration entered	Shows error about duration

UI State Persistence

#	Test Case	Expected Result
7	Set hours=1, mins=30, select sets 1 & 3, activate. Reopen Override	Hours, mins, and checkbox selections are remembered

Override Behavior

#	Test Case	Expected Result
8	Activate override for set 1, visit blocked site in set 1	Site is accessible
9	Activate override for set 1 only, visit blocked site in set 2	Site is still blocked
10	Activate override, check toolbar icon	Icon changes to override icon (orange)
11	Wait for override to expire OR cancel override	Icon returns to normal

Edge Cases

#	Test Case	Expected Result
12	Activate override for set 1, then go to Options and disable "Allow temporary override" for set 1	Override continues until expiry (matches lockdown behavior)
13	Activate override for sets 1 & 3 for 5 mins, then activate for set 2 for 10 mins	Set 1 & 3 have 5 min override, set 2 has 10 min override (independent)
14	Use "Cancel Override" from popup menu	All active overrides are canceled

Override Limit (if configured)

#	Test Case	Expected Result
15	Set override limit to 2/day, activate override for 3 sets at once	Counts as 1 use (not 3)
16	Reach override limit, try to open Override	Shows limit reached error