Adding custom filter subscriptions (by URL or file path)

[ameshkov]

There should be a possibility to add custom filter subscription by URL or file path.

Requirements

1. Add custom filters to the same group as user filter.
2. Handle the situation when users adds custom filter we already have (like EasyList) (use subscription_url field to merge them).
3. Filter parser should handle "Homepage", "Version", "Expires" fields
4. We should properly handle filters without such metadata.

If "Homepage" is not set - do not display it.
If "Version" is not set - display something like "Version from 15/01/2015"
Important: Handle empty version on update check result screen.

Parse "Last modified". If not set - use DateTime.Now.
If "Expires" is not set - set it to 48 hours.
Parse "Title". If "Title" not set - use URL instead (cut it to 64 characters)

1. Ignore [Adblock Plus 2.0] line

UI

1. New button to "Add filter" dialog:
   http://screencast.com/t/5W0DuQyo
2. New Filter Subscription dialog:
   http://screencast.com/t/THZmkmbSsPg

User may enter abp:subscribe or subscribe.adblockplus.org URL.
We should handle it.

[gshumihin]

Fixed