Refactor that fixes a compilation issue in Release mode

Some template specialization were not found, because they were not declared
at the point they were used.

We moved things around, things are less inter-dependant, and also now it
works.

src/database/insert_query.cpp

@@ -0,0 +1,21 @@
+#include <database/insert_query.hpp>
+
+template <>
+std::string before_value<Database::Date>()
+{
+  if (Database::engine_type() == DatabaseEngine::EngineType::Sqlite3)
+    return "julianday(";
+  if (Database::engine_type() == DatabaseEngine::EngineType::Postgresql)
+    return "to_timestamp(";
+  return {};
+}
+
+template <>
+std::string after_value<Database::Date>()
+{
+  if (Database::engine_type() == DatabaseEngine::EngineType::Sqlite3)
+    return ", \"unixepoch\")";
+  if (Database::engine_type() == DatabaseEngine::EngineType::Postgresql)
+    return ")";
+  return {};
+}