Improve event note display in dashboard and project activity views

Fixes #3910 

This change improves the event note display in dashboard and project activity views:

* Preserve link href values when the link text is truncated.
* Display inline code in the first line of a note.
* Only display "..." when the note contents are truncated.
* If the note starts with a code block, display the first line instead of an empty `<pre/>` element.

### Examples

#### Old

![truncated_link_old](https://cloud.githubusercontent.com/assets/2551678/4609238/8e47f9f4-5299-11e4-8283-aac0d64bd58a.png)
---
![inline_code_old](https://cloud.githubusercontent.com/assets/2551678/4609240/969e7772-5299-11e4-9e5e-8051ac074d33.png)
---
![plain_old](https://cloud.githubusercontent.com/assets/2551678/4609241/9b7c9648-5299-11e4-9e64-b29c199f9495.png)
---
![code_block_old](https://cloud.githubusercontent.com/assets/2551678/4609243/9fb3dcda-5299-11e4-9b17-d2f1783d4a7b.png)

#### New

![truncated_link_new](https://cloud.githubusercontent.com/assets/2551678/4609272/36feca50-529a-11e4-8712-51ae5f2a5e58.png)
---
![inline_code_new](https://cloud.githubusercontent.com/assets/2551678/4609273/3fb6bfa4-529a-11e4-8109-1670bbdc058e.png)
---
![plain_new](https://cloud.githubusercontent.com/assets/2551678/4609276/4485838a-529a-11e4-8574-7034eab88f68.png)
---
![code_block_new](https://cloud.githubusercontent.com/assets/2551678/4609277/490b53ee-529a-11e4-95c2-3ad0657b2e19.png)