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/db/entity/InstanceEntity.kt

@@ -15,6 +15,7 @@
 
 package com.keylesspalace.tusky.db.entity
 
+import androidx.room.ColumnInfo
 import androidx.room.Entity
 import androidx.room.PrimaryKey
 import androidx.room.TypeConverters
@@ -41,6 +42,8 @@ data class InstanceEntity(
     val maxFieldNameLength: Int?,
     val maxFieldValueLength: Int?,
     val translationEnabled: Boolean?,
+    // ToDo: Remove this again when filter v1 support is dropped
+    @ColumnInfo(defaultValue = "false") val filterV2Supported: Boolean = false
 )
 
 @TypeConverters(Converters::class)