PLT-3105 Files table migration (#4068)

* Implemented initial changes for files table

* Removed *_benchmark_test.go files

* Re-implemented GetPublicFile and added support for old path

* Localization for files table

* Moved file system code into utils package

* Finished server-side changes and added initial upgrade script

* Added getPostFiles api

* Re-add Extension and HasPreviewImage fields to FileInfo

* Removed unused translation

* Fixed merge conflicts left over after permissions changes

* Forced FileInfo.extension to be lower case

* Changed FileUploadResponse to contain the FileInfos instead of FileIds

* Fixed permissions on getFile* calls

* Fixed notifications for file uploads

* Added initial version of client code for files changes

* Permanently added FileIds field to Post object and removed Post.HasFiles

* Updated PostStore.Update to be usable in more circumstances

* Re-added Filenames field and switched file migration to be entirely lazy-loaded

* Increased max listener count for FileStore

* Removed unused fileInfoCache

* Moved file system code back into api

* Removed duplicate test case

* Fixed unit test running on ports other than 8065

* Renamed HasPermissionToPostContext to HasPermissionToChannelByPostContext

* Refactored handleImages to make it more easily understandable

* Renamed getPostFiles to getFileInfosForPost

* Re-added pre-FileIds posts to analytics

* Changed files to be saved as their ids as opposed to id/filename.ext

* Renamed FileInfo.UserId to FileInfo.CreatorId

* Fixed detection of language in CodePreview

* Fixed switching between threads in the RHS not loading new files

* Add serverside protection against a rare bug where the client sends the same file twice for a single post

* Refactored the important parts of uploadFile api call into a function that can be called without a web context

store/sql_upgrade.go

@@ -181,7 +181,6 @@ func UpgradeDatabaseToVersion33(sqlStore *SqlStore) {
 
 func UpgradeDatabaseToVersion34(sqlStore *SqlStore) {
 	if shouldPerformUpgrade(sqlStore, VERSION_3_3_0, VERSION_3_4_0) {
-
 		sqlStore.CreateColumnIfNotExists("Status", "Manual", "BOOLEAN", "BOOLEAN", "0")
 		sqlStore.CreateColumnIfNotExists("Status", "ActiveChannel", "varchar(26)", "varchar(26)", "")
 
@@ -199,6 +198,9 @@ func UpgradeDatabaseToVersion35(sqlStore *SqlStore) {
 	sqlStore.GetMaster().Exec("UPDATE ChannelMembers SET Roles = 'channel_user' WHERE Roles = ''")
 	sqlStore.GetMaster().Exec("UPDATE ChannelMembers SET Roles = 'channel_user channel_admin' WHERE Roles = 'admin'")
 
+	// The rest of the migration from Filenames -> FileIds is done lazily in api.GetFileInfosForPost
+	sqlStore.CreateColumnIfNotExists("Posts", "FileIds", "varchar(150)", "varchar(150)", "[]")
+
 	// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 	// UNCOMMENT WHEN WE DO RELEASE
 	// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!