Use Intl to localise dates and times (#11422)

* Use Intl to generate better internationalised date formats

* Get `Yesterday` and `Today` from Intl also

* Correct capitalisation blunder

* Fix formatTime include weekday

* Iterate

* Fix tests

* use jest setSystemTime

* Discard changes to cypress/e2e/settings/general-user-settings-tab.spec.ts

* Discard changes to res/css/_components.pcss

* Discard changes to res/css/views/elements/_LanguageDropdown.pcss

* Discard changes to src/components/views/elements/LanguageDropdown.tsx

* Add docs & tests for getDaysArray & getMonthsArray

* Discard changes to test/components/structures/__snapshots__/MatrixChat-test.tsx.snap

* Consolidate consts

* Improve testing & documentation

* Update snapshot

* Apply suggestions from code review

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

* Iterate

* Clarify comments

* Update src/DateUtils.ts

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

* Specify hourCycle

* Discard changes to test/components/views/settings/devices/DeviceDetails-test.tsx

* Update comments

---------

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

src/languageHandler.tsx

@@ -93,7 +93,7 @@ export class UserFriendlyError extends Error {
 
 export function getUserLanguage(): string {
     const language = SettingsStore.getValue("language", null, /*excludeDefault:*/ true);
-    if (language) {
+    if (typeof language === "string" && language !== "") {
         return language;
     } else {
         return normalizeLanguageKey(getLanguageFromBrowser());