Make reading time field editable

[vbh]

Currently reading time field is only readable, this PR adds a reading time field in the Add/Edit book screen which will be visible only if the status is finished. If reading time is not edited or set then the default logic of reading time (calculated based on start and finish dates) will be used.

[mateusz-bak]

Could you fix the conflicts? It doesn't looks like a trivial change and didn't want to add it 2.1 but now I can take a look for 2.2.

[mateusz-bak]

Ok I reviewed it now, sorry it took long time.

1. I think the custom reading time should be saved as milliseconds instead of the string.
2. The custom time button should be displayed below start/end date when editing book.
3. There's some unnecessary space when editing with in progress and unfinished book statuses.
4. The statistics should take the new reading time into considerations - that's why i think the milliseconds field will work the best.

I'm doing some other stuff with the db etc and I'm happy to work on the changes I mentioned. Just let me know if you are okay or if you want to implement them yourself @vbh

[mateusz-bak]

Looks good, just small mentioned changes