[chore] tidy up media manager, add calling func to errors, build-script improvements (#1835)

* media manager tidy-up: de-interface and remove unused PostDataFunc

Signed-off-by: kim <grufwub@gmail.com>

* remove last traces of media.Manager being an interface

Signed-off-by: kim <grufwub@gmail.com>

* update error to provide caller, allow tuneable via build tags

Signed-off-by: kim <grufwub@gmail.com>

* remove kim-specific build script changes

Signed-off-by: kim <grufwub@gmail.com>

* fix merge conflicts

Signed-off-by: kim <grufwub@gmail.com>

* update build-script to support externally setting build variables

Signed-off-by: kim <grufwub@gmail.com>

---------

Signed-off-by: kim <grufwub@gmail.com>

internal/processing/account/account.go

@@ -34,7 +34,7 @@ import (
 type Processor struct {
 	state        *state.State
 	tc           typeutils.TypeConverter
-	mediaManager media.Manager
+	mediaManager *media.Manager
 	oauthServer  oauth.Server
 	filter       *visibility.Filter
 	formatter    text.Formatter
@@ -46,7 +46,7 @@ type Processor struct {
 func New(
 	state *state.State,
 	tc typeutils.TypeConverter,
-	mediaManager media.Manager,
+	mediaManager *media.Manager,
 	oauthServer oauth.Server,
 	federator federation.Federator,
 	filter *visibility.Filter,