This makes it easier to separately handle search and index requests
from a web server or from a reverse proxy.
If a request to index contains a query, a permanent redirect HTTP response
is returned. This should give some level of backwards compatibility
for users that have set a searx instance in their browser's search bar.
The new url parameter "timeout_limit" set timeout limit defined in second.
Example "timeout_limit=1.5" means the timeout limit is 1.5 seconds.
In addition, the query can start with <[number] to set the timeout limit.
For number between 0 and 99, the unit is the second :
Example: "<30 searx" means the timeout limit is 3 seconds
For number above 100, the unit is the millisecond:
Example: "<850 searx" means the timeout is 850 milliseconds.
In addition, there is a new optional setting: outgoing.max_request_timeout.
If not set, the user timeout can't go above searx configuration (as before: the max timeout of selected engine for a query).
If the value is set, the user can set a timeout between 0 and max_request_timeout using
<[number] or timeout_limit query parameter.
Related to #1077
Updated version of PR #1413 from @isj-privacore
