Partial revert "Replace uses of config_get() by calls to plugin_config_get()"

This partially reverts commit 9222599ab5f04615763262a0b8784d40cf7a2590.

It is required that we use config_get() referencing the 'Source' plugin
in SourceIntegration.php. Because SourceIntegration is a child plugin of
Source, plugin_config_get() does not refer to the correct config string
when retrieving the setting, causing 'Configuration option not found'
errors to occur.
Added option to select view state of bug notes

Prior to this, the 'Bug fix' notes view state was hardcoded to public.
We are now allowing the administrator to choose whether they should be
public or private by means of a config option.

Fixes #42

Cyril's original commits were squashed.

Signed-off-by: Damien Regad <damien.regad@merckgroup.com>
Use Selection list to pick bugfix notes view state

To be consistent with MantisBT core, the bugnote view status is now set
by means of a selection list (i.e. pick public/private) instead of a
checkbox.
Added explanatory text for private bugnotes setting

bugnote_add() API will force VS_PUBLIC state if changeset's author does
not have private_bugnote_threshold.
Move text formatting for some language strings to config page
Updated French translations