Allow for searching for blog posts

[cdriehuys]

Users should be able to search for blog posts by title and content. In GraphQL form, the query would look like:

query {
  posts(search: "Foo") {
    title
  }
}

Acceptance Tests

1. Searching by title

Given posts with titles "Foo" and "Bar", searching for "Foo" should only return the post titled "Foo".

2. Searching by body content

Consider the scenario with the following posts:

Post(content='Bar', title='Foo')
Post(content='Baz', title='Bang')

If the user searches for 'Bar', the first post should be returned since 'Bar' is contained in the content.

3. Search precedence

Give two posts, one with the title "Foo" and one with the content "Foo", searching for "Foo" should return the post with title "Foo" first since titles have higher search precedence.

[cdriehuys]

This is blocked until we have a plain text version of the post's content that we can search through. See #1.