Change description of privacy levels to be more intuitive in web UI (#13197)

* Improve description of privacy levels in compose interface

* Change strings in defaultMessage and source as well as english

Co-authored-by: Thibaut Girka <thib@sitedethib.com>

app/javascript/mastodon/locales/defaultMessages.json

@@ -1073,7 +1073,7 @@
         "id": "privacy.public.short"
       },
       {
-        "defaultMessage": "Post to public timelines",
+        "defaultMessage": "Visible for all, shown in public timelines",
         "id": "privacy.public.long"
       },
       {
@@ -1081,7 +1081,7 @@
         "id": "privacy.unlisted.short"
       },
       {
-        "defaultMessage": "Do not show in public timelines",
+        "defaultMessage": "Visible for all, but not in public timelines",
         "id": "privacy.unlisted.long"
       },
       {
@@ -1089,7 +1089,7 @@
         "id": "privacy.private.short"
       },
       {
-        "defaultMessage": "Post to followers only",
+        "defaultMessage": "Visible for followers only",
         "id": "privacy.private.long"
       },
       {
@@ -1097,7 +1097,7 @@
         "id": "privacy.direct.short"
       },
       {
-        "defaultMessage": "Post to mentioned users only",
+        "defaultMessage": "Visible for mentioned users only",
         "id": "privacy.direct.long"
       },
       {