Various cleanups to Ubuntu installation instructions:

* Avoid editing /etc/apt/sources.list add nginx repo, instead create
  /etc/apt/sources.list.d/nginx.list

* Avoid editing /etc/nginx/nginx.conf, instead put server_name_hash_buckets
  setting into discourse.conf

* Proper-case Discourse when it is referring to the product name, so that all
  the lowercase references are to the user created during the installation
Address @Supermathie's concerns in PR1430
Updated Japanese locales
Optionally allow discourse to create new topics from email.
Refactor and clean up New-Topic via Email

With the new email_in admin configuration setting, emails to the email_in_address fetched via POP will now be processed and posted as new topics to the forum.

With the email_in_min_trust you can control the trust level the user needs to have at least to be able to post an email as a new topic.

Also contains tests for the email-in feature and minor clean ups
Add Email-In-Per-Category

 - allow the configuration of an inbox-email-address per category
 - post emails to that email into that category instead of global

 - Adds UI for configuration
 - Adds Documentation for configuration
 - Adds Tests for new feature
Add public-inbox to Email-In-Feature

 - Adds the advanced option to accept email from non-users per category email-address
 - Adds tests covering the new feature
 - Adds UI to configure this feature in the frontend
Switch to proper exception handling system for better user feedback

 - Replace implicit return code-system in Email::Receiver with proper exception system
 - Update tests to check for exceptions instead
 - Test the PollMailbox for expected failures
 - Add proper email-handling of problematic emails
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Dropping squashing commits from contribution process

 - Rephrases squashing of commits before submitting from a requirement to a recommendation
 - minor fixes around that
add better new forum image
hey valid HTML is always a nice bonus
Explicit button text in abandon dialog in all lang

The current post abandon dialog displays typical yes/no buttons and should
instead show more informative button text.

This commit updates all `config/locales/client.*.yml` files and *alters the
structure* of the `post.abandon` node, and *adds three new nodes* under it:
`confirm`, `yes_value`, and `no_value`. The current value of the
`post.abandon` node is moved into `post.abandon.confirm` for all languages.
For English language text, the nodes `post.abandon.yes_value` and
`.no_value` are changed to `"Yes, abandon"` and `"No, keep"`, respectively.
For other languages, the standard yes/no values are copied, allowing simple
copyedits to make them more explicit in the future.

I also make the necessary changes to the `composer_controller.js` file so
the new values are used correctly.
add mobile screenshots to readme
let vision stay on discourse.org
rel="prev" and rel="next" for topic also
add swap link
omit needless words, again
Typo fix
minor updates
move requirements into a table for EZ readin'