Merge branch 'maintenance' into devel

Conflicts:
	src/octoprint/static/js/app/viewmodels/files.js
	src/octoprint/util/pip.py

CONTRIBUTING.md

@@ -90,7 +90,9 @@ following section *completely*. Thank you! :)
    the people maintaining them instead.
 
 2. Please make sure to **test out the current version** of OctoPrint to see
-   whether the problem you are encountering still exists.
+   whether the problem you are encountering still exists, and **test without
+   any non-bundled plugins enabled** to make sure it's not a misbehaving
+   plugin causing the issue at hand.
 
    If you are feeling up to it you might also want to try the current development
    version of OctoPrint (if you aren't already). Refer to the [FAQ](https://github.com/foosel/OctoPrint/wiki/FAQ)

src/octoprint/util/pip.py

@@ -370,6 +370,10 @@ class PipCaller(CommandlineCaller):
 	def _preprocess_lines(self, *lines):
 		return map(self._preprocess, lines)
 
+	@staticmethod
+	def _convert_lines(lines):
+		return map(PipCaller._convert_line, lines)
+
 	@staticmethod
 	def _preprocess(text):
 		"""