only check once for filters v2 availability (#4539)

Instead of calling the endpoint every time filters are needed, it will
be called only once and the result cached. This will result in quite
some requests less on instances supporting v2.

I also tested v1 filters and made some small improvements. We should
[remove filters v1
support](https://github.com/tuskyapp/Tusky/issues/4538) some time in the
future though.

app/src/main/java/com/keylesspalace/tusky/entity/FilterV1.kt

@@ -28,13 +28,6 @@ data class FilterV1(
     val irreversible: Boolean,
     @Json(name = "whole_word") val wholeWord: Boolean
 ) {
-    companion object {
-        const val HOME = "home"
-        const val NOTIFICATIONS = "notifications"
-        const val PUBLIC = "public"
-        const val THREAD = "thread"
-        const val ACCOUNT = "account"
-    }
 
     override fun hashCode(): Int {
         return id.hashCode()