Fix image description unnecessarily html-escaping innocent characters (#321)

* implement SanitizeCaption function

* tidy up text test setup

internal/text/caption.go

@@ -0,0 +1,29 @@
+/*
+   GoToSocial
+   Copyright (C) 2021 GoToSocial Authors admin@gotosocial.org
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU Affero General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Affero General Public License for more details.
+
+   You should have received a copy of the GNU Affero General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+package text
+
+// SanitizeCaption runs image captions (or indeed any plain text) through basic sanitization.
+// It returns plain text rather than HTML, in contrast to other functions in this package.
+func SanitizeCaption(in string) string {
+	content := preformat(in)
+
+	content = RemoveHTML(content)
+
+	return postformat(content)
+}