feat(aio): use shorter URLs for previews

Use the 7 first characters of the 40-chars long SHAs for shorter/cleaner URLs.
The collision probability is extremely low (since all SHAs are further
"namespaced" under the corresponding PR). In case of a collision, the second PR
will not be deployed, in order to avoid overwriting the original build.

(This is a design decision to keep the implementation simple. It can be changed
later if necessary.)

aio/aio-builds-setup/dockerbuild/scripts-js/lib/common/constants.ts

@@ -1,2 +1,3 @@
 // Constants
 export const HIDDEN_DIR_PREFIX = 'hidden--';
+export const SHORT_SHA_LEN = 7;