Unsupported bleve index version

[lafriks]

When upgrading gitea from unsupported bleve index version gitea does not start and fails with errors:

2017/09/17 00:43:04 [...els/issue_indexer.go:21 InitIssueIndexer()] [E] Unable to open issues indexer (indexers/issues.bleve). If the error is due to incompatible versions, try deleting the indexer files; gitea will recreate them with the appropriate version the next time it runs. Deleting the indexer files will not result in loss of data.
2017/09/17 00:43:04 [...les/indexer/issue.go:61 InitIssueIndexer()] [E] InitIssuesIndexer, open index: incompatible version, 7 is supported

In such case I think index file should be automatically deleted and regenerated

Introduced in PR #2387 @ethantkoenig