Add shell chapters, update functions lesson, handle missing Welcome.md

[DergunNik]

Regular Expressions and Special Commands sed,awk,grep,sort chapters were added for shell in English. The Shell Functions lesson was updated.

Welcome.md for Spanish shell was also added, since es already had the Hello World! lesson, but without an appropriate Welcome.md file the server would not boot. This issue existed before my changes, so the current version from GitHub could not be run.
The server failed with the following error (from docker):

... other warnings
app-1  | WARNING:root:File learnshell.org/en/Users_Management.md is not a tutorial
app-1  | Traceback (most recent call last):
app-1  |   File "/usr/src/app/main.py", line 351, in <module>
app-1  |     init_tutorials()
app-1  |     ~~~~~~~~~~~~~~^^
app-1  |   File "/usr/src/app/main.py", line 197, in init_tutorials
app-1  |     tutorials.remove("Welcome.md")
app-1  |     ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^
app-1  | ValueError: list.remove(x): x not in list
app-1 exited with code 1

It may be worth adding an if check at line 196 in main.py to prevent similar cases in the future:

  if "Welcome.md" in tutorials:
      tutorials.remove("Welcome.md")
      tutorials = ["Welcome.md"] + tutorials
  else:
      logging.warning("Welcome.md not found in tutorials for %s/%s", domain, language)