gui, lib/config, lib/model: Allow absolute values for minimum disk free space (fixes #3307)

This deprecates the current minDiskFreePct setting and introduces
minDiskFree. The latter is, in it's serialized form, a string with a
unit. We accept percentages ("2.35%") and absolute values ("250 k", "12.5
Gi"). Common suffixes are understood. The config editor lets the user
enter the string, and validates it.

We still default to "1 %", but the user can change that to an absolute
value at will.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4087
LGTM: AudriusButkevicius, imsodin

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

@@ -68,6 +68,7 @@
    "Editing {%path%}.": "Editing {{path}}.",
    "Enable NAT traversal": "Enable NAT traversal",
    "Enable Relaying": "Enable Relaying",
+   "Enter a non-negative number (e.g., \"2.35\") and select a unit. Percentages are as part of the total disk size.": "Enter a non-negative number (e.g., \"2.35\") and select a unit. Percentages are as part of the total disk size.",
    "Enter comma separated  (\"tcp://ip:port\", \"tcp://host:port\") addresses or \"dynamic\" to perform automatic discovery of the address.": "Enter comma separated  (\"tcp://ip:port\", \"tcp://host:port\") addresses or \"dynamic\" to perform automatic discovery of the address.",
    "Enter ignore patterns, one per line.": "Enter ignore patterns, one per line.",
    "Error": "Error",
@@ -242,6 +243,7 @@
    "This Device": "This Device",
    "This can easily give hackers access to read and change any files on your computer.": "This can easily give hackers access to read and change any files on your computer.",
    "This is a major version upgrade.": "This is a major version upgrade.",
+   "This setting controls the free space required on the home (i.e., index database) disk.": "This setting controls the free space required on the home (i.e., index database) disk.",
    "Time": "Time",
    "Trash Can File Versioning": "Trash Can File Versioning",
    "Type": "Type",