Use relative URLs, not absolute.

This helps if somebody would fork a GitHub repository, as the links
now lead to files in fork, not files in original repository. Also, it
makes more sense when reading Markdown files locally.
Add hint on how to rebake posts
Add Post#is_first_post?

We should be able to ask a post if it's the first in a topic
Add Topic#max_post_number to find max post number
Add Topic#url for determining url for a Topic
Add test to verify posts are moved in transaction
Simplify user action make it more idiomatic
Merge pull request #895 from GlitchMr/readme-links

Use relative URLs, not absolute.
Merge pull request #845 from Supermathie/rebakedocs

Add hint on how to rebake posts
introduce strong_parameters
Extract PostMover from Topic into its own class
Merge pull request #903 from chrishunt/chrishunt/extract-post-move

Extract post move