Added the ability to define a custom footer via environment variables

Added the CUSTOM_FOOTER_TEXT and CUSTOM_FOOTER_URL environment variables.

If undefined, the default translated footer will display.

If only CUSTOM_FOOTER_TEXT is defined, only this defined text will display in place of the normal footer text.

If only CUSTOM_FOOTER_URL is defined then the defined URL will display.

If both variables are defined, the defined text will display as a link to the defined URL.

server/clientConstants.js

@@ -13,6 +13,8 @@ module.exports = {
     FOOTER_CLI_URL: config.footer_cli_url,
     FOOTER_DMCA_URL: config.footer_dmca_url,
     FOOTER_SOURCE_URL: config.footer_source_url,
+    CUSTOM_FOOTER_TEXT: config.custom_footer_text,
+    CUSTOM_FOOTER_URL: config.custom_footer_url,
     COLORS: {
       PRIMARY: config.ui_color_primary,
       ACCENT: config.ui_color_accent