added skeleton bountie page telling no bounties available

i2p2www/pages/site/get-involved/bounties.html

+{% extends "global/layout.html" %}
+{% block title %}{{ _('Bounties') }}{% endblock %}
+{% block content %}
+<p>{% trans -%}
+Thank you for your interest in contributing to I2P!<br>
+<br>
+Currently there is no bounty system available.
+<br>
+{%- endtrans %}</p>
+{% endblock %}
+
+