Process optimisation

<table>
  <tr>
    <th>Q</th><th>A</th>
  </tr>
  <tr>
    <td>Bug fix?</td><td>yes</td>
  </tr>
  <tr>
    <td>New feature?</td><td>no</td>
  </tr>
  <tr>
    <td>BC breaks?</td><td>yes</td>
  </tr>
  <tr>
    <td>Tests pass?</td><td>yes</td>
  </tr>
  <tr>
    <td>License?</td><td>MIT</td>
  </tr>
</table>

- Fix `@fm_bbcode.decoda_manager` service
- Use a default locale message if the message does not exists on the current locale
- Add `fm_bbcode.resources` configuration key (is a list of config include paths)
- Make `fm_bbcode.config.messages` optional