internal links: Inline external logic to `getLinkType` and simplify.
internal links: Inline external logic to `getLinkType` and simplify.

Take all the logic from the `isGroupLink`, `isTopicLink`,
`isStreamLink`, and `isSpecialLink` and inline it in `getLinkType`.

This saves on few extra function calls and makes the code more
straightforward.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile