chore: add global definitions for paths to simplify includes

[mcdurdin]

Adds _includes/prepended_header.php which defines _KEYMANCOM_INCLUDES and _KEYMANCOM_COMMON, which are then available to every .php page on the site.

Fixed up all require/require_once references to use the appropriate defines consistently. There remain some require_once references that do not link _KEYMANCOM_* -- these are for other files that are not under either of those include folders.

Note: regex (require_once|require|include|include_once)\b(?! _KEYMANCOM) will find all include/require[_once] references (and a few irrelevancies). Removing the local refs (e.g. to session.php in keyboards/) leaves the following files which don't use these paths and could be further cleaned up in a future PR -- but these are low priority and low risk:

7 results - 4 files

_includes\autoload.php:
   2:   require_once __DIR__ . '/../vendor/autoload.php';
   9:         $success = include(__DIR__ . "/locale/{$filename}.php");
  11:         $success = include(__DIR__ . "/2020/{$filename}.php");
  24:       $success = include(__DIR__ . "/../_common/{$filename}.php");

_includes\2020\Util.php:
  19:           require_once($_SERVER['DOCUMENT_ROOT'].'/cdn/deploy/cdn.php');

_includes\includes\servervars.php:
  19:   require_once __DIR__ . '/../../vendor/autoload.php';

_includes\locale\Locale.php:
  86:         self::$strings[$domain][$locale]-> strings = require $currentLocaleFilename;

Test-bot: skip

[keymanapp-test-bot]

User Test Results

Test specification and instructions

User tests are not required

[mcdurdin]

Skipping the /downloads/releases/ paths shortcuts a lot of tests. There's a risk there if we change those pages of course.

[darcywong00]

Do we adjust the link checker in the GHA too?

keyman.com/.github/workflows/ci.yml

Lines 40 to 45 in 947c553

	# Exclude checking locales for each link 'lang=*'
	run: |
	set +e
	set +o pipefail
	npx broken-link-checker http://localhost:8053/_test --recursive --ordered ---host-requests 50 -e --filter-level 3 --exclude '*/donate' --exclude '*lang=*' | tee blc.log
	echo "BLC_RESULT=${PIPESTATUS[0]}" >> "$GITHUB_ENV"

[mcdurdin]

Do we adjust the link checker in the GHA too?

I'll do that as a separate PR, perhaps. In some ways it's better to keep the separate one.

I saw something that concerned me with the link checker -- it seemed to continue to operate with pages with server errors. I will have to review to make sure we aren't missing those.

[darcywong00]

lgtm

[darcywong00]

Do we adjust the link checker in the GHA too?

keyman.com/.github/workflows/ci.yml

Lines 40 to 45 in 947c553

	# Exclude checking locales for each link 'lang=*'
	run: |
	set +e
	set +o pipefail
	npx broken-link-checker http://localhost:8053/_test --recursive --ordered ---host-requests 50 -e --filter-level 3 --exclude '*/donate' --exclude '*lang=*' | tee blc.log
	echo "BLC_RESULT=${PIPESTATUS[0]}" >> "$GITHUB_ENV"