feature: get problems by category

[viveknathani]

you can tap onto any category from the link under the problem in the list of problems but apparently, it is not accessible or obvious.

what do users say:

1. "add a section wise search"
2. "it's cumbersome to first look for a particular type of question and then specifically clicking for the little text below"
3. "is there a filter for topic?"
4. "and if I want to change the category I need to go back?"
5. "a side bar which slide opens with the list of categories"
6. "what happens when I click on complex numbers?"