ISSUE_TEMPLATE.md: Nudge contributors to specify episodes unambigously

See https://github.com/swcarpentry/r-novice-inflammation/issues/401 > 3.

Fix https://github.com/carpentries/workshop-template/issues/540

See apple.com/macos

Code/callout/prereq/etc blocks: set left and right margins to 0

* replace page.root with 'base' variable that knows path level

* deal with case of trailing / in URL

* use link to figure out path for logos

* simplify path building for favicons

* baseurl isn't used in our template so it can removed
* use 'base' variable for path root instead of unecessary http request to
  site.url

* replace page.root with 'base' variable that knows path level

* deal with case of trailing / in URL

* use link to figure out path for logos

* simplify path building for favicons

* baseurl isn't used in our template so it can removed
* use 'base' variable for path root instead of unecessary http request to
  site.url

* rename variable, consolidate comments

* rename variable in template files

Pull Request carpentries/styles#330
Closes carpentries/styles#343

1. Fix the script so that all figures are displayed.
2. Add image numbers.
3. Don't add headers for episodes without images.
4. Add horizontal rulers to better separate adjacent images.
   Don't add a ruler after the last image in the episode.
5. Add empty lines to `figures.md` to improve code readability.
6. Use classes for styling text and figures

Pull Request: carpentries/styles#327

Pull Request: carpentries/styles#216

Pull Request: carpentries/styles#323

Close carpentries/styles#325

added workshop calendar

space out callout elements better

Fix lesson_initialize.py to initialize new lessons