Create / remove search index logic

When you create a search index, then you can only delete it if you want to recreate it again.

But, if there is no search index, and a post is created, then the post is indexed and in acp it show like if the search index is created, you can't index the previous posts without delete the search index.

I have added a few checks to launch an exception or just do nothing when there is an index already created to avoid that, but it causes some errors in the tests.

I think that only the posts should be indexed if there is an index created, if there isn't an index, the posts should not be indexed. Explained in other way: Have all posts indexed or none, but not only the last posts created since the index was cleared the last time.