Introduce GiteaLocaleNumber custom element to handle number localization on pages. (#23861)

Follow #21429 & #22861

Use `<gitea-locale-number>` instead of backend `PrettyNumber`. All old
`PrettyNumber` related functions are removed. A lot of code could be
simplified.

And some functions haven't been used for long time (dead code), so they
are also removed by the way (eg: `SplitStringAtRuneN`, `Dedent`)

This PR only tries to improve the `PrettyNumber` rendering problem, it
doesn't touch the "plural" problem.

Screenshot:


![image](https://user-images.githubusercontent.com/2114189/229290804-1f63db65-1e34-4a54-84ba-e00b44331b17.png)


![image](https://user-images.githubusercontent.com/2114189/229290911-c88dea00-b11d-48dd-accb-9f52edd73ce4.png)

modules/util/util_test.go

@@ -224,10 +224,3 @@ func TestToTitleCase(t *testing.T) {
 	assert.Equal(t, ToTitleCase(`foo bar baz`), `Foo Bar Baz`)
 	assert.Equal(t, ToTitleCase(`FOO BAR BAZ`), `Foo Bar Baz`)
 }
-
-func TestDedent(t *testing.T) {
-	assert.Equal(t, Dedent(`
-		foo
-			bar
-	`), "foo\n\tbar")
-}