Import redmine_logs plugin

Import the redmine_logs plugin code by Haruyuki Iida as a starting point.
The code is imported from: https://bitbucket.org/haru_iida/redmine_logs
Commit id: 0046f89d303b
Rename plugin to redmine_log_viewer

Rename the plugin to redmine_log_viewer and update the plugin's metadata
with new information.
Only list log files

Only list the log files in the log file directory. Those are files whose
extension is either .log or .log.X, where X is one or more digits. The
latter case is for displaying old, rotated log files.
Add tooltip to log file link

Add an HTML title attribute to the log file link to create a tooltip
explaining that it's not the entire log file that is shown.
Remove log deletion capability

Remove the capability to delete log files from within the administration
interface. Log files should be handled externally to the application
instead, e.g. using logrotate.

Removing this capability ensures Redmine administrators cannot remove
log files (unless they are also server administrators). This is good
from an audit perspective.
Correct link to stylesheet in show view.
Add UTF-8 encoding to init.rb file
Fix typo
Sort log files by timestamp instead of path
Remove download-button from show-view

It wasn't working anyway, so just remove it. Also provides a cleaner ui
with only one place to download the log file from.