[PURR][#2816]Allow knowledge base articles to be accessible through API

[kuang5]

PURR Ticket: https://purr.purdue.edu/support/ticket/2816

We want to expose the knowledge base articles through API so that we can the external knowledge base wiki application can use the API to get the knowledge base articles on PURR to keep the content at external application synchronized.

The API endpoint is /api/v1.0/content/list?limit=10&limitstart=0, where limit is the number of the records to return and the limitstart is pagination offset. A third parameter category is also available in case the query is restricted to certain category articles.

Test procedure:

1. Visit https://dev.purr.purdue.edu/api/v1.0/content/list?limit=10&limitstart=0 and the response should includes fields including success status, total records, limit, start, and the content.

The test is done on dev PURR and it works as expected.