Create Entry Model and Build Sample Data

This adds an Entry class modeled from the spiked proof-of-concept. This
object has the following fields:

* User ID: The ID of the user triggering the entry. Used in looking up
  entries by user.
* User Name: A display name for the given user.
* Happened At: A timestamp of when the action occurred.
* Action Type: A unique identifier for the action that occurred (e.g.,
  creating a page, logging in, or anything else). Used in looking up
  actions across users, as well as indicating how to interpret the
  metadata.
* Metadata: Arbitrary keys and values associated with the entry.
  Interpreted based on Action Type (e.g., '{id: 123}' when creating a news
  article can mean something different from '{id: 123}' when creating an
  account)

QA Plan
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1. `rake db:setup`
2. `rake db:seed`
3. `rails console`
4. `Entry.count` should be 10000.
5. `Entry.first` should show a valid entry.