cmd/syncthing, gui, lib/config, lib/upgrade: Add option to upgrade to pre-releases

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3939

gui/default/assets/lang/lang-en.json

@@ -26,6 +26,7 @@
    "Be careful!": "Be careful!",
    "Bugs": "Bugs",
    "CPU Utilization": "CPU Utilization",
+   "Candidate releases": "Candidate releases",
    "Changelog": "Changelog",
    "Clean out after": "Clean out after",
    "Close": "Close",
@@ -81,6 +82,7 @@
    "GUI Authentication Password": "GUI Authentication Password",
    "GUI Authentication User": "GUI Authentication User",
    "GUI Listen Addresses": "GUI Listen Addresses",
+   "GUI Theme": "GUI Theme",
    "Generate": "Generate",
    "Global Changes": "Global Changes",
    "Global Discovery": "Global Discovery",
@@ -120,6 +122,7 @@
    "Newest First": "Newest First",
    "No": "No",
    "No File Versioning": "No File Versioning",
+   "No upgrades": "No upgrades",
    "Normal": "Normal",
    "Notice": "Notice",
    "OK": "OK",
@@ -181,6 +184,7 @@
    "Single level wildcard (matches within a directory only)": "Single level wildcard (matches within a directory only)",
    "Smallest First": "Smallest First",
    "Source Code": "Source Code",
+   "Stable releases only": "Stable releases only",
    "Staggered File Versioning": "Staggered File Versioning",
    "Start Browser": "Start Browser",
    "Statistics": "Statistics",
@@ -234,6 +238,7 @@
    "Upgrading": "Upgrading",
    "Upload Rate": "Upload Rate",
    "Uptime": "Uptime",
+   "Usage reporting is always enabled for candidate releases.": "Usage reporting is always enabled for candidate releases.",
    "Use HTTPS for GUI": "Use HTTPS for GUI",
    "Version": "Version",
    "Versions Path": "Versions Path",