Replace `link_to_gfm` with `link_to` in merge request and issue titles.

## What does this MR do?
This removed GitLab Markdown Formatting from issue titles due to issues and confusion it caused as well as performance reasons.

On the issue/MR page itself, the formatting is preserved. This only effects index pages. See:

![Screen_Shot_2016-05-24_at_3.31.17_PM](/uploads/8857ed7f3cc6a47ea41ca88134e5d17a/Screen_Shot_2016-05-24_at_3.31.17_PM.png)

I think the trade-off here is worth it, otherwise we'd just be playing wack-a-mole.

## Are there points in the code the reviewer needs to double check?
I don't think so.

## Why was this MR needed?
Previously if an issue had an emoji for its name it would be unclickable. Similarly, links were rendered inline if the title was something like “Fixes a bug with https://google.com”. This confused a number of users, so it’s removed.

This is also ever so slightly more performant :)

## What are the relevant issue numbers?
This fixes #17614 and #17230.

## Screenshots (if relevant)

Before:

![Screen_Shot_2016-05-24_at_3.19.52_PM](/uploads/25313af3d3995a096b7bad7c5a26d071/Screen_Shot_2016-05-24_at_3.19.52_PM.png)

After:

![Screen_Shot_2016-05-24_at_3.20.13_PM](/uploads/f14f09918001bcbac2cd939b016479b2/Screen_Shot_2016-05-24_at_3.20.13_PM.png)

Before:

![Screen_Shot_2016-05-24_at_3.20.48_PM](/uploads/c183ae2c491650c328eb5768f6a55fd9/Screen_Shot_2016-05-24_at_3.20.48_PM.png)

After:

![Screen_Shot_2016-05-24_at_3.20.42_PM](/uploads/d3431a691768bae2c5244b9b632061b9/Screen_Shot_2016-05-24_at_3.20.42_PM.png)

cc: @rspeicher @jschatz1 

See merge request !4284