Added SSL Validation Override and CA Selection

- Added fields to login page to a) disable ssl validation or b) select
  a custom Certificate Authority certificate to use with the server.

- Changed visibility of widgets on login page from INVISIBLE to GONE so
  they don't take up space while hidden (since this was causing weird
  spacing issues with the new fields).

- Added state to settings to store ssl validation choice or certificate
  data.

- Added fields to various HTTP methods to disable ssl validation or set
  valid certificate authority if either setting is enabled.

app/src/main/res/values/strings.xml

@@ -31,6 +31,14 @@
     <string name="gotify_url">Gotify URL</string>
     <string name="username">Username</string>
     <string name="password">Password</string>
+    <string name="show_advanced">Show Advanced Options</string>
+    <string name="disabled_validate_ssl">Disable SSL Validation</string>
+    <string name="select_ca_certificate">Select CA Certificate</string>
+    <string name="select_ca_file">Select a Certificate File</string>
+    <string name="or">OR</string>
+    <string name="please_install_file_browser">Please install a file browser</string>
+    <string name="select_ca_failed">Failed to read CA: %s</string>
+    <string name="no_ca_selected">No CA selected</string>
     <string name="login">Login</string>
     <string name="check_url">Check URL</string>
     <string name="gotify_logo">gotify logo</string>