img naming convention and organization

In response to Issue #125 a [translation workflow guide](https://github.com/AmericanRedCross/Guides/blob/master/TranslationWorkflow_LearnOSM/readme.md) is being developed.

This pull request contains edits of the English version with the intention of making the process of creating a new translation easier.

#### images:
- consistent use of reference style image syntax in chapters (previously a mix of reference and inline)
- language subfolder within images folder
- file naming convention:  
en_beg_[two digit chapter #]_  
[chapter name based on permalink]_  
image[two digit # code]_  
[image name based on image reference id]
- {{site.baseurl}} removed from file paths
    - images didn't load for All Chapters on live site
    - this change (when running the site locally) has images loading on the single chapter pages and on the All Chapters page

**NOTE:** The chapter and image numbers are not a good system and hard to update. But for now, all the files in
"images\en\" are being used on the published pages (no old files not being used), the numbers are paired with text descriptors, the files are all in one place, and the organization is consistent (making batch renaming easy with code or any number of utilities).
**NOTE:** The jekyll liquid filters are still displaying after the last post for All Chapters. Not sure what the fix is.

#### other (unrelated):
- link in *CONTRIBUTING.md* updated to skip a redirect