[db] validate repo certificate

for now only for initial repos, but eventually we should validate for CoreRepository as well to prevent invalid certs from entering the DB by other means.

libs/database/src/dbTest/java/org/fdroid/database/RepositoryDaoTest.kt

@@ -32,7 +32,7 @@ internal class RepositoryDaoTest : DbTest() {
             address = getRandomString(),
             mirrors = listOf(getRandomString(), getRandomString(), getRandomString()),
             description = getRandomString(),
-            certificate = getRandomString(),
+            certificate = "abcdef", // not random, because format gets checked
             version = Random.nextLong(),
             enabled = Random.nextBoolean(),
             weight = Random.nextInt(),