Article filter dialog improvements

[MrOrz]

1. Add "OK" button to mobile menu

From 20200729:

手機上 filter 按 X 會以為是 cancel，實驗之後才發現是按下去就會 apply

Figma: "Apply filter" https://www.figma.com/file/zpD45j8nqDB2XfA6m2QskO/Cofacts-website?node-id=2322%3A0

We can retain the behavior that pressing filter buttons starts filter process immediately (because it's the current behavior and is easier to implement), but add an "OK" button at the bottom of filter dialog so that user can click to close filter dialog.

2. Better highlight color for selected items

https://g0v.hackmd.io/n74W64y4RiWIKt1Noxn-Mg#%E5%A4%A7%E6%9D%BE%E6%AA%A2%E8%A8%8E

Should change the border of the selected item into primary yellow color.